Thanks everyone for your responses.
Lots of useful resources to look into with DSS and it's good to have some starting points as he is in a bit of a spin with it all and emotions are high.
To clarify a few things... I am referring him to DSS as he is my partners eldest child, however he was 19 when I met my partner so I have no maternal involvement. I'm keen to support him alongside my DP and there have been no shortage of tough love conversations where we (and his mum) are adamant he needs to put babies needs first.
For those who are aghast at the idea of this baby being away from his mum for more than a couple of hours... as a couple they left baby overnight with both her mum and his mum so they could go out from approx 4 weeks old. They had nights in hotels to have a "break" and also nights on the town and not returning til lunchtime the following day with hangovers and both mums happy to continue to care for baby whilst they slept it off. (I have soooooo many opinions about this...I EBF my sons until they were 30 months, had them in a sling most of the time, embraced 4th trimester and didn't spend a night apart from them until eldest was 3 and 3 months when I was in hospital after having his baby brother so I am forever biting my lip on the choices that they have made) Despite my judgy pants being firmly hoiked, I can see that DSS is very loving and caring daddy and the idea that he is in anyway less capable of caring for the baby than his mum is not the case.
DSS moved to his GF's city and had a job. They lived with her mum and so when the relationship broke down he was asked to leave. He doesn't like the city, hasn't made any friends apart from workmates and her friends and did make a snap judgement when she threw him out to quit his job and come home.
He's currently on his mum's sofa and feeling very low.
He needs a job and to then get his license but that isn't going to happen overnight.
I think short-term the letter suggested by one of the first posters is a good starting point and what I will gently recommend, despite being 24 DSS and his ex are pretty immature and she is using baby as weapon which is not in their best interest at all.
I can see it getting messy if I'm honest. They hadn't been together long really when baby was conceived accidently, her mum doesn't like him and has a lot of background that leads DSS's mum to think that she is pulling the strings (5 kids, 4 dads, ex-drug addict, long-term benefits etc) and her single daughter with a baby is now far more likely to get a HA flat than as a couple. I've only met her once and all our knowledge is secondhand from DSS and his mum. I generally try not to judge without my own experience but the optics aren't great!