Money to DS not DIL

A car would count as a joint family asset. Paying his rent would keep his debt down and benefit his wife when the financial split was eventually made. You need proper legal advice. Do you have a Citizens Advice Bureaux you could visit?

A car is also an asset and you have to estimate its value on the financial form so cars and possessions up to a certain value are included. I'm not sure about rent

Thanks - that is what I suspected. At the moment he seems reluctant to do anything formally as they are managing to agree child access etc fairly amicably and he doesn`t want to rock the boat.
What would happen if we did something like occasionally paying his rent? Or even replacing his car?

Agree.
You could perhaps offer to e.g. clear any debts or his credit card now but do not give him the money until he is free.

What TwiceAsNice said.

Until your son is legally divorced any money belongs to both of them and has to be declared. They both have to fill in a very detailed financial assets form and it has to be declared in court. If it is found that either had money they didn't include there are heavy legal penalties including the possibility of prison. I suggest he gets divorced as soon as possible and you give him some money after that. If you give it before it is not all his.