as he's type 1 and can therefore adjust his insulin, it's really not essential to make a specific 'diabetic' cake. our diabetes nurse always used to say birthdays, christmas and other special times were for indulging yourself and you won't come to any harm. when my son (who's had type 1 since he was 10)was still living at home, we just used to have ordinary birthday cake and he would have a couple of extra units of insulin if he'd eaten a lot of cake. if he'd only had one piece, he didn't even bother doing that.
as spangle says, diabetes uk has some good recipes if you really want to make a 'diabetic' cake, although it's people with type 2 diabetes who have to be very strict with their diet.