Oh dear this is very hard for you i can tell.
You have bonded very well and it hurts to leave him. I worked in kennels and we often had dogs that just came for the day. At first it might be abit stessfull for you both but after a few weeks he will look at it as part of the routine and settle down.
Do they exercise the dogs together as we found that the dogs we had looked forward to coming and ran in through the doors once they had got used to the routine. I think they looked forward to seeing who was in.
Don't feel guilty as you have to work and he will benefit from meeting other dogs and people.

I know it all seems hard now but persevere and you will find that the dog will benefit by being with others rather than left on its own.
The other alternative is to hire a dog walker

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Grandma to Jinja Dog, Mum to Tilly the Cavalier, Tattycat, Moomy, Choo, Sonic, Niven Mc Twisp and Lawna the rabbits and 1 guinea,17 budgies, 4 Cockatiels, 8 Goldfish.