OP - You need to think carefully before marrying this man. He is financially insecure, and I think it is dangerous for you to tie yourself legally to someone who is unable to support himself without your help. Marriage is something you should do when he is capable of contributing to it, not just all on you. It seems very telling of your situation that you and your parents are paying for it, and he isnt - kind of like your domestic situation as a whole.
Are you prepared for being a step parent. It isnt easy at all. You are committing to having this little boy, and his mother in your life for the long haul. You will never come first to your DP, your needs and wishes will always come second, can you handle that? You seem lovely, and I speak from someone who has been through hell, with a wet lettuce of her own. The fact that you are willing to contribute to your partners son is lovely, but he needs to have the means to do it himself.
You have been roasted on here, so I'm just going to say please please think long and hard before getting married. I'm not saying leave him, but its a commitment I don't think he should be making before he can offer you some security. By security I mean, being equally able to contribute to your life together. The money your parents are spending on a wedding, would be better spent on you - learning to drive, or holiday, or a secret get out fund. I'm not saying he is a bad person, but he doesnt seem to be able to be relied upon. Never leave yourself without security and options. Take it from someone who married and had kids with a cocklodger and is still cleaning up his mess, after we have split.
I do think he needs a serious chat with a careers advisor, to properly explore his options. The OU is fantastic and is a sensible option. He needs the means to support himself as well as his kid, and needs to be able to do so without you. A bit of time properly exploring his options will not be time wasted. I would spend some time researching places locally that can assist with job searching, CVS etc. If he is applying and searching and having no joy, something is up. His CV may need work, etc.
Ignore his ex and her situation, no matter how annoying, it will do nothing but make you bitter. Sit down, have the talk with him, and together come up with a viable, workable plan where he can study but earn more. He may need to give up this course, or embark on a different one, as I suspect he isnt getting any luck, due to him not having clear availability like other jobseekers.
I hope you dont see my post as an attack, as I think sometimes a bit of support is needed, but you need to look at your situation from an outsiders point of view. You are only treated the way you allow yourself to be treated, ffs, dont let that be like a bank with unlimited credit, or basically his Mum.
Big hugs xx