Well I think you have done well to keep him in for 6 months, I know it can be difficult especially when they pine to go out.

The best advice I can give you when you are starting to allow a cat outside for the first time, is to build them up slowly, to be honest, you may very well find that they are scared of funny things like birds flying over to start with. Start with short supervised play times outside when the weather is good, for about 10 – 15 mins, build it up to longer periods over a few days. After about a week, let him out for half an hour or so by himself, but do not feed him before he goes out. Hungry cats want their dinner so they won’t go far. Build his outside playtimes up gradually.