What are they attributing this to? Can he not be referred to see a physio OP? Or indeed a specialist physio if he hasnt got anywhere so far. Tight muscles can usually be improved by prescribed exercises (from sitting/standing) and stretching to target them.
Re: pain has he seen dedicated pain management? You can ask to be referred to them or the community pain team or even occupational therapy if he needs mobility aids. What has been tried? There are a lot of options, combinations of meds and playing around with doses when it comes to pain relief, the NHS offers alternative therapies also. It can take years to find what works. The goal is to reduce pain to make things like more exercise more manageable, nothing will likely erase it.
I was chronically ill as a teen, and am even more so as an adult. I understand his anxieties and frustrations but as you say, he has a girlfriend so clearly 'someone will want him', as will many others. I've not ever had any issues finding or retaining lovely supportive partners as a result of not being able to walk far (among other apparently unattractive things 
). Not everyone is ableist, even if the society we live in is.
Reduced mobility or capacity is not no capacity. I'm sure there are plenty of things he can do, they just may be more challenging and require thought or planning, allowances made, use of mobility aids to prevent fatigue or pain, working towards a kind of job that could be done from home. He is certainly not alone in being unwell as a young person and perhaps it would help him to speak with others via online forums.
What would have helped me as a teen would have been for my parents to believe me, understand I lack capacity and accommodate it instead of hoping it will go away, a sense of calm and to support me with advocating medically for more tests and a full diagnosis and generally being more positive. My DM did and still does spend the vast majority of the time distraught at the state of me and expressing her sadness at how my life has turned out and I have essentially removed her from my life as I don't share the sentiment and find her draining. Life does go on when you are chronically unwell and dwelling on differences to normal people and difficulties faced rarely helps. Illness may happen to any of us at any point.
There is nothing stopping you from speaking to your own GP for support with your anxiety. These things are hard and knowing how to cope for the best is difficult.