Sounds like a really horrible situation all round OP.
I always applaud anyone who tries to further themselves through education, so well done to your DP for taking this step, it takes courage. However, in the kindest possible way...have you both really talked about what it will take for him to become a physiotherapist? My daughter is one, I'm a teacher, and I know how tough it is for even the top percent of A* students to become one.
First, you have to have 5 GCSEs, which include English, Maths and AT LEAST one Science at grades A-C. So that's going to take him several years, if, at this stage, he's actually struggling to write.
Then, in order to even stand a chance of getting onto the massively hard to get on Physio degree course (which you have to do to become one) you need 2 (or even 3 for some) A levels, which include a Biological Science and PE.
So by now we are on a MINIMUM here of 5 years.
Then the degree! Which is a really tough, science based degree. Full time is 3 years, part-time anything from 4-6 years. If your OP needs to work during this he would have to choose p/t as the actual full time course is very intense with block placements, so would be difficult to fit in a job that paid enough to support a family.
So you're looking at a minimum of 8 years, possibly 10 or 11 before he's even ready to register with the HCPC...which is a requirement to practice physiotherapist.
I'm not saying all this to make you (or him) feel bad. But it does sound like you are both not really thinking long term. or making any realistic decisions about your future.
Learning to write (and read to a really good standard, if this is also an issue) should be priority. Your DP should still be working though, whilst doing this. Anything else, all those idealistic dreams, have to take a back seat until he passes 'Phase 1: learn to write'.
Your mum is probably very frustrated at how your DP appears, to her, to not be supporting you at all. And I think I'd feel the same to be fair. (I'd not act like she does though!)