What would you do?

My 4 almost 5 year old who is developmentally delayed son's father wasn't in his life for the first 2 years (his choice I tried believe me I tried.)...then he came around I let him in he would be around a month maybe 4 then gone for a month or so...this time around it's been almost 2 years since my son last seen him he wrote today asking to see him that this time around he was here to stay he was clean blah blah (which I never knew he had a drug problem)....my fiance does not want him back around as my son just started calling him dad again hes been around since day 1 and my son called him dad until bio dad came around and has stopped asking about bio dad. I want him in my sons life but I also don't want to keep hurting my son.

I wouldn’t. He has a habit of just leaving and the older your son gets the more he’ll notice & it’ll hurt his feelings. It sounds like the person you’re with is a lot better