He is very much not a random person
Ok so hopefully nothing too awful you don't know about - although you still need to be cautious as you don't know his son or his ex.
Plus people with an excellent reputation outside of their home/intimate relationships can be very different in them. I'd still say don't assume you're being given the full story and that there aren't very good reasons why the son doesn't want to see the dad.
It may be something as "benign" as dad has constantly let him down as a parent - promising big and not delivering, cancelling contact last min, not making much effort during contact etc
I've had that with my dd. My ex would describe the situation as me preventing him seeing dd. The truth is I bent over backwards for many years practically, emotionally and financially to facilitate and even encourage contact with dd and her dad, even when she didn't want to (the reasons for which I sadly didn't discover until much later and included under feeding her, ignoring her pointedly, infantilising her, not allowing her to speak to me while on contact - we live hundreds of miles apart as he's ex army so contact needed to be overnight. After a particularly bad contact dd insisted I stay locally thereafter so I was paying out for hotels etc too).
There came a point where by coincidence dd discovered all I'd been doing due to her dad slipping up and I was struggling financially after becoming too ill to work. Dd kinda sat ME down for a conversation and said that she didn't want me killing myself to facilitate contact and she wanted to know once and for all if he was interested in being her dad and so she asked me to back off from all I'd been doing (this included things like booking and paying for his travel as well as hers and/or doing and paying for the travel myself, repeatedly reminding him when her school holidays were and reminding him to book his leave from work, reminding him of travel arrangements, reminding him of her birthday etc etc)
So I did - it was bloody hard and many times when seeing how heartbroken dd was I was tempted to do a "secret" text or call telling him to bloody well be a father!
What happened?
Within a year he was no longer even phoning her! Hell even if she phoned him he couldn't get off the call fast enough!
Her birthday card was weeks late and inappropriate age wise, no gift or even money to get herself something.
A few months after that in response to her asking him (politely not angry, more hurt) why he was treating her so badly - he blocked her on everything! Yep! He blocked his own daughter!
But to hear him tell it (he's an idiot who forgets we still have friends in common) I told dd a load of lies and stopped her visiting him. I did nothing of the sort - there was no need he was perfectly capable of screwing it all up himself!
I've never blocked him, dd never blocked him, dd has a disability which was diagnosed around the same time which I kept him and his parents informed about. I'm still in contact with his mum. Dd has had hospital admissions and other stuff with her disability which I have messaged and written to him about. One time she was very seriously ill in hospital, I text him (I'll admit I withheld number as by this stage he had me blocked too) and the reply I got?
"Thanks for letting me know"
No concern, no asking how she was or asking to be kept informed, no call to dd (I'd given him the ward details if he didn't want to call mobile, I also gave him the names of the dr and named nurse in case he wanted to speak to them) he did NOTHING! He couldn't give a shit!
But I'm the bad guy! 
And my dd was left thinking her dad barely cared if she lives or dies! 


But the impression he gives people who don't know me or dd is that he was a wonderful, caring dad who was ripped out of his dds life by evil me.
I hear he's been caught out on this BS a few times doesn't stop him trying it.
He can't get away with it with wife 2 though as she and I were friends before their affair and we stay in touch and she knows exactly what happened and she argued with him about his shitty attitude too. It's the main reason she won't divorce him herself as even though they're now miserable together she fears he would treat their dc similarly