Yes, my DS was diagnosed with a brain tumour 2 years ago and surgery and treatment have left him with ataxia, colour blindness, partial deafness, processing difficulties etc. He is also an ambulatory wheelchair user.
He has two lovely friends from before the brain tumour, who are fabulous, but they don't go to the same school and, due to their extracurricular activities, we don't get to see them that much. His close friends from school do still invite him to their birthday parties, although a recent one we have declined because it's a sport-based party that he just couldn't physically do. Both the child and mum were lovely about it, with both suggesting DS go and watch from the sidelines, so he could join in with the cake afterwards, but it would have meant an hour sitting by himself watching other people have fun, so he decided he wouldn't go. When he first became well enough, his close friends still invited him over for playdates, even though it was practically difficult as people here have small flats/houses and the wheelchair takes up an enormous amount of space. But that has tailed off and when a big group go to the park after school, DS is often left standing on the sidelines. This is the same in the playground at school whenever I have walked past. The kids are also starting to make fun of DS's physical differences, which is hard.
So...I would say the success story is his original two friends that he's known since he was a baby. The rest have become 'friends for a season', as the saying goes. I am sad on his behalf, and I also worry that as the processing difficulties become more apparent with age, he'll struggle even more to fit in with a group. I am also looking for groups outside of school, although I'm searching based on personal interest rather than ability (e.g. things like archery, which he's already interested in, can be done standing or from a wheelchair). Are you based in London? Do you go to the annual GOSH sports day? That's a great opportunity for the kids to mingle with those in similar situations and try a wide range of activities, like wheelchair basketball, tennis, climbing, cycling, archery etc.
Can your DS use his right leg at all? Could something like cycling (on a tricycle or with training wheels) be something he could do with mates?
P.S. Come over to the children with cancer board, where you may find others who have experienced similar.