You could maybe investigate some clubs he might like, and think of ways of using the DLA to fund those; perhaps a carer to accompany him, if relevant.
The idea is to bridge the gap between what he is able to access ordinarily, and what he could access were the funding to be there to remove the barriers to participation. He should have the opportunities that other young people of his age access. I'm sure you are focused on making sure that happens, from what you say, and organisations to help families experiencing disability - like Mencap - could suggest what other families do with the funding.
In some cases, children with disability require more stimulation and activity as they get older and reach the teenage years, and want interests which don't involve being taken places with the family. I don't know your son's age, but you might find his leisure needs increase over the coming years, and then it will be very clear what needs to be funded to enable him to live the life he wants to.
Clubs, and learning a musical instrument; types of physical activity are things he might want to do, going forward.. It is sometimes difficult to access group sessions, and the DLA ( PIP now) is useful to fund one-to-one instructed sessions. My son, who is autistic, did climbing and kayaking and tennis and skateboarding and swimming, and always had one-to-one instruction, as accessing a group session wouldn't have worked. He is probably a lot more into physical activity than many. There was a time when he was primary-aged when going to the park would have been enough, aside from playing with toys and having cycling and swimming lessons, but, in the teen years, he wanted more. Mentors are a good idea for older teens, as it gives the young person someone to relate to and talk to who isn't a family member, and if it's a young, cool mentor, it often boosts the self-esteem and makes the young person excited about reaching adulthood.
Just be prepared for things to change. Have the savings there. Make sure there are some holidays geared to what he likes. Pay for after-school clubs. School will put in his mind activities which he might like to take up in his leisure time. ( They are always trialing BMX and climbing and things). Just be conscious of always giving him the chance to express a preference for what to do, and you should be ok, because you are clearly conscientious and thinking about how to spend the money.