Ok that may be the case but controversial take here.
If he is in university with Jayce and John he will have to be pretty close age wise. So lets assume he is 17 the first half of the VN won't be NSFW anyway. And until the later half of the VN David could have his birthday and be 18 totally legal.
I played another game called Tennis Ace where one of the Love interests starts out as 17 and the MC is 19 and they start dating when he still was 17. They do only cute stuff (for all the love interests) in the beginning and only late during the second half of the routes does anything remotely NSFW happen. And by then the love interest was 18.
And it's nothing weird about it its not like the MC took advantage of him they were friends who just happened to be some ages apart but not as big an age gap than to call it pedophilia.
Disclaimer this is just an idea and not meant to tell Monchi what to write. Again it's his VN if he doesn't want there to be a David route there won't but the possibility is there.
If we assume Davids route could be a branch off of Maccoons route it could go like his.
David: "I like you go and fuck my brother"
Jayce:"Nuu too nervous and he is so sexy he would never want me"
They start becoming friends and David continues to try hook up Jayce with his bro.
But slowly Davids feelings change he and Jayce while trying to hook up Jayce with Mac have bonded over alot of stuff and then when Jayce is about to actually fuck/ask out Mac David realizes his feelings.
Drama ensues. Maybe David would feel he isnt man enough for Jayce because Jayce clearly has a thing for Mac. Maybe the concerns about the age gap could be brought up by Mac himself he likes Jayce but doesnt like David dating Jayce his overprotective brotherly side won't allow it. Maybe after Jayce gets over Mac and actually considers David Tom swoops in and confesses for real (maybe pushed by Mac because again Mac doesnt want David to be dating Jayce) and Jayce would of course consider that too because Tom has been very good friends with Jayce and there is more history there. There are alot of possible events that have nothing sexual about it before David needs to be of age. And all that could take like several weeks. Unless Monchi wants to keep it by a day by day basis then as convoluted as it sounds just make Davids birthday during one of these days. Or if that doesnt work maybe keep David as a completely vanilla sfw route. Not as popular but you dont need sex for love to exist.
And again assuming David and Jayce are close enough age wise. And in a fictional setting age is an arbitrary number Monchi as the creator can decide which ages they are, so even if David is 16 right now he can just say he is 17 and later on he turns 18. Nobody can say thats wrong because Monchi is the creator.
I know there is a stigma against aging up characters to sexualize them but come on pairs with age gaps exists and especially if its still after High School in University and they were friends before actually developing feelings then this is just a normal relationship like any other.
So yeah it is possible to write a David route without making it creepy its just up to Monchi.