Given he's likely presented you with the best version of what happened when dss was younger and you already concede dh's contact with dss was 'patchy' and he 'could have tried harder' I'm guessing he was as useless as my ex - who was in the army until v recently.
I was an army brat myself and have relatives and friends still in, some of whom are nrp's - and who make a damn site more effort than my ex, or by the sounds of things your dh did. But I've also come across other instances of nrp's in the forces who use it as an excuse to fade out their kids lives.
As pp have noted - nrp's need to make an extra effort toward their bond with their child/ren BECAUSE they're not doing the daily graft of parenting - the bonus of this being the child knows that parent is ALWAYS there for them.
It's not JUST about direct contact either, and the forces infrastructure recognises this and provides support for absent parents to maintain indirect contact with their families. There's a variety of methods set up which I won't go into that allows service people to stay in touch with family 'ban home'. There's certainly no excuse for not doing so when he was home. Doesn't sound like your dh did this with any real commitment either. As in my ex's case I wonder just how hard is it REALLY to write/call/text regularly. (Clue: it's not hard if you make it a priority).
What you seem to be working really hard to avoid recognising is that your dh HURT his son by not being an involved and interested father - and not just once but REPEATEDLY over several years. Many adults would really struggle to cope with that kind of CONSTANT rejection let alone a child.
I've had to pick up the pieces each time dds been told her dad is too busy to make a weekly phone call, or text, let alone use annual leave (which the army gets a pretty good deal on) to actually see her. Even when he was having contact, I was doing most of the transport!
Because what the dad in this scenario is really saying to THEIR CHILD is - 'you matter less than my career, my new family, my time off, my time I want to spend doing hobbies - basically everything else in my life. You're bottom of my list of priorities' - how would you feel op? If your dh treated you like that? I know if an op came on here and says their dh was doing this they'd be told to LTB!
"those children we see a lot of." So he COULD have seen 'a lot of' dss too. And his being 'young' is no excuse either! Old enough to make a baby - old enough to take responsibility for it! Dss's mum certainly had to - and did!
"I am trying to persuade him to get it out in the open and for DH to talk to DSS how he feels etc so they can move on with their adult relationship on an equal footing but it’s like trying to crack a nut." Probably because dh KNOWS what would be coming at him from dss if he opens that can of worms!
He was/is a shit dad, you may not like hearing that - doesn't make it less true.
"and had nowhere for DSS to physically stay." If dh told you this that's bollocks! There's quarters set aside for PRECISELY this purpose, we've used them and I know several other families who have too. Going back 30+ years to my recollection.
"He needs telling with a sharp word, it's rude, whoever is doing it." Dreadful advice pretty much guaranteed to make dss go Nc altogether!
"And he cannot write while he is away." According to whom?
"One where you can’t have any contact with them while you are away."
A these are extremely rare! Most roles there's secure communication portals set up for serving people to communicate with their loved ones. It's not done completely altruistically - service folk who feel well supported in maintaining home links make more effective motivated soldiers/sailors/air personnel.
B people in these roles do other things eg writing 'letters' which they can't send at the time but act as a sort of diary which they do regularly and then send when they're back at base. Or write letters ahead of leaving to be sent at agreed periods to their family members and loved ones
C if he REALLY were in a role like this - he wouldn't even be allowed to tell you, let alone you say so on a public forum!
"Or it's just convenient to say that but isn't actually true" - exactly! Someone's bullshitting somewhere along the line.
For now - an honest conversation where the father honestly admits ALL his past failings is one thing, and may start the healing of the relationship, having a go at the dss is the absolute worst way to go.
Dh also needs to recognise it may take many years to repair the rift HE caused with HIS actions.