Just reading your OP again.....
DH also paying for therapy for him as he has a lot of guilt at not being with him as he grew up.
It looks like you're not aware of the issues his DS is dealing with...as many dads leave, but the fact that his DS is engaging in therapy indicates there's probably a lot you're not aware of. He may have confided about some of his issues and therapy was suggested. It's good that he's seeing a professional.
You mention your DH feels guilt.... maybe some of the issues with hus DS are as a result of this. Divorce/family split should not mean his dad wasn't around for him...but it sounds like that was the case.
Perhaps his DS was scared to make a commitment with his GF...as buying the house is the first step and usually marriage or kids or kids and marriage...either way round.
That may well be a trigger for his own upbringing and bring up the feelings of abandonment he felt when his dad left.
Think about this from your DH's perspective for a moment.....
Your DH left his Ex/relationship ended while his DS was very young, so while your H was and is present for your DC.... he didn't see his first child much after the split....my estimation is his DS was between 3 - 5 years old when his dad left....perhaps your DH feels £1k a month for 8 months in the grand scheme of things (his absence and the impact on his DS) is a small amount to pay.
It doesn't sound like his DH has been a strong source of support to his DS growing up and sees this as his opportunity to redeem himself as a father and be there, as he wasn't in his younger years.
He has now spent some of the money on a car he doesn’t need.
How do you know he doesn't need it? Is the car sitting there unused?
At least it's something tangeable.