Why don't you have a look locally and see if you can find a decent dog training class? It would be a very good way to socialising your young dog in a controlled environment.
If you tell us roughly whereabouts you are, one of us may know of a suitable class in your area

 

I am in Suffolk, IP21 5RB

 

Twiggy suggested exactly what I would suggest, a dog training class is the perfect place where it is safe/secure and under control, he and you get to socialise with other dog owners/dogs and where he will learn to contain himself

 

He sounds like he would benefit from some impulse control training.

 

Certainly, training classes will be good for him, but don’t expect them to miraculously solve the ‘problems’. The reality is - through no fault of yours, obviously - that your puppy has missed the important socialisation opportunity (up to about four months old) when he should have been learning how to interact with other dogs. I hate to seem to be a doom-monger (and I’m not trying to be, just doing a bit of ‘expectation management) , but there it is. It should get better as he gets older, matures and calms down a bit - but the fact is that there’s a window in which socialisation (and habituation) needs to happen to be effective, and your dog is well past that. I shouldn’t worry too much, though - he WILL (well, SHOULD ) calm down a bit, but you will likely need to work on that. His interactions with other dogs may not be great, but that can be managed as well.

 

Certainly, training classes will be good for him, but don't expect them to miraculously solve the 'problems'. The reality is - through no fault of yours, obviously - that your puppy has missed the important socialisation opportunity (up to about four months old) when he should have been learning how to interact with other dogs. I hate to seem to be a doom-monger (and I'm not trying to be, just doing a bit of 'expectation management) , but there it is. It should get better as he gets older, matures and calms down a bit - but the fact is that there's a window in which socialisation (and habituation) needs to happen to be effective, and your dog is well past that. I shouldn't worry too much, though - he WILL (well, SHOULD ) calm down a bit, but you will likely need to work on that. His interactions with other dogs may not be great, but that can be managed as well.

That is an extremely negative post and it could well drive a lesser mortal into abandoning their dog. Personally I do not agree with your doom laden prognosis and that is fortunate for my pooch I guess. If there are others reading that post I would recommend caution. There are many who disagree with the above.

 

Regardless of if you adopted this dog now or had it from a few weeks old and didn't do people/dog socialisation at the puppy stage ( up to 4 months old), Dogs are ALWAYS prepared to change their behaviour as long as their owner is prepared to change their behaviour... you have an 8 month old, so a teenager and even the best trained/socialised pups once they hit teenage time will often behave like human teenagers do, sweet and loving one minute and 'horrid henrys' the next minute. My advice is set your goal, go and get professional hep in a dog training class where they will take you through sessions that you would not have the confidence to try yourself, the natural competition with other dogs/handlers will spur you on to try new things and you will gain the confidence on how to deal with your teenage pup/dog.... put the work in with him and it will get much better...one step at a time.

 

@Ian246 isn't being 'doom laden'.

It is science that puppies have 'learning windows' up to a certain age, during which they learn stuff like how to behave with other dogs, strange people, excitement and stress.

These 'windows' close and if experiences are missed out on then it's not possible to present them in the future with the same 'soaking it up like a sponge' effect.

That said, there's no reason you won't get over this issue but it might be expected to be harder work than with a pup who had a more sociable start than lockdown.

Training classes will be great for you; start trying now to work out why your pup is behaving like this.
Is it fear (I'll make a huge kerfuffle and hopefully that scary dog won't want to come any closer), or does he see another dog as a source of entertainment? Or whatever?

In the meantime, decide what is the distance away from another dog that he doesn't react ie close enough to be aware of it but not so close that he goes loopy, and reward the calmer behaviour.

Also perhaps decide what is his most favourite reward/thing to do WITH YOU, so that you can eventually become more interesting than other dogs.

If you can avoid too many other dogs, for now and until you can get this under control a bit, then so much the better: Any behaviour which is repeated is strengthened and learned. That's how we train dogs after all.

Hope this has been of some help.

cross posted