my puppy is showing aggression but only around me?

I have a 13 month old Patterdale springer spaniel cross. he is extremely high energy but we're giving him lots of exercise and enrichment I was almost obsessive about socialising him when we got him and he loves other dogs most of the time. while he is my dog technically, due to work my dad is the one who cares for him most of the time and he babies the pup aggressively and has no issues while on walks.

I take him out on the same walks the same routes and I tend to be sterner with him and he listens but for the past 3 months or so he goes for other male dogs. I thought it might be because he isn't neutered yet but my dad doesn't have the issue and on occasion hes gone for a female dog as well but under different circumstances (the dog was sniffing around his ball)

I dont understand what it is im doing wrong or why he responds so differently with me.