@PinkDaydreams what about a hot chocolate buffet? If he doesn't like hot choc you can just do milk.
Make up hot chocolate in the slow cooker or on the hob
Put out a selection of toppings in little dishes - squooshy cream, honeycomb, smarties, chocolate flakes, sprinkles, cinnamon, edible glitter.
Then you just go to town and let him make up his own
If there's two of you or more you can still do pass the parcel and pin the tail on the donkey.
Definitely a balloon disco. A friend of mine used to go to town with birthdays for her three kids, but even now as giant teens all they remember are the living room balloon discos
I'd also take him a walk with a birthday boy badge on, possibly clutching a balloon, because round here you'd be guaranteed to have people stopping and wishing him happy birthday, even in these desolate times. And it doubles up as positive social interaction, plus little kids bloody love telling every man and his dog that it's their birthday