If you are looking for hard evidence to show mark did something, or prove it in court, you aren't going to find it here.
But if you are looking for clues that make all the other possibilities far less likely:
1. Phone goes dead at 8pm, and never gets turned back on - even to check text messages.
2. Friends get stood up, and Dylan doesn't try contacting them to reschedule. No phone, email, or text.
3. Dylan is wearing shorts and a t-shirt - and it's not mild weather for hiking.
4. Cadaver dogs, air scent dogs, multiple ones, hit on the lake.
5. Father has history (albeit very infrequent if more than once) of violence involving sons.
6. Father JUST lost custody in a 7 year battle, and had to fight for the visitation. He probably wouldn't be scheduled to see Dylan again for a few months (usually the other parent gets xmas, and then you swap the next year). That's expensive as well as heartwrenching and possibly angering.
7. Father gets home and doesn't worry that Dylan is not there. No note, no text, and dylan will not answer his phone. His friends are well over 30 minutes drive away, potentially in a slightly different direction, so it's not like he just stated walking there hoping to cross paths. Dylan's father doesn't call/text the mom or police until after dark (I didn't verify the time of sunset in the area, so maybe not quite dark yet?).