Welcome DIK brother
I lean towards Neil not being the MC's bio dad as well, but it's a very messy subject/theory because DPC has messed with the timeline before (changing when the MC met Zoey I believe) which means any speculation about ages not lining up etc. can be yanked out from under us. It's fun to mess around and theorize though.
The evidence that tends to sway me are the themes of the game surrounding blood and family; the first lines of the game are a retrospective by the MC about family being the people who support you and all that good stuff, as told to him by Neil himself, but how exactly is that relevant to the MC's story when Neil is then immediately introduced as a super great biological dad who has never let him down? The theme of 'Family' is literally the first word of the game, but the MC is actually the only main character in the game who has a relatively drama-free biological family. Maya, Josy, Jill, Bella, Sage, Quinn - all of them have been shown to have problems with their families, their dads, moms, adoptive parents, sisters, husbands etc. The theme of 'family' fits all of them much more than the MC, except for the potential of a big secret in Lynette's diary - but what possible secret could be relevant to the theme of 'family being those who support you and not those who share your blood'? Because again, that theme fits almost everyone except the main character of the story, the guy who literally said those words - what could make those words relevant from him, who already has a fantastic biological dad - the best dad the story has shown us?
Only makes sense to me thematically if Neil isn't his real dad. Otherwise the game should start up with the MC saying something like: "My dad Neil always said family was those who supported you instead of those who shared your blood, which is weird because he's my real biological dad who I love the most in the world, and the only other person who I might have considered non-biological family left my ass to go to San Diego and broke my heart, oh and plus I really really wanna know about my biological mom, so actually this isn't relevant at all"
Anyhow that's just my prediction - Neil isn't the MC's dad and he'll have to struggle with what family means once he finds out who his 'true' family is and that Neil's been lying. Makes for good drama and DPC is all about that, and besides I can't even imagine any other family secret having half as much impact on the MC as him finding out that Neil isn't his bio dad. Maybe Lynette being alive or something, but then we'd have to add the incest tag and that would be pure anarchy
