I don't think anyone has said anywhere that the OPs son has to break away completely from his family 🙄. Him beginning to have his own life with a girlfriend, work and friends outside of the family is completely normal and healthy.
I see these posts crop up time and time again with this belief that "family and ONLY family" stops people being lonely in old age. I work in adult social care and this couldn't be further from the truth. I've met elderly people from all family sizes and it doesn't stop them from being lonely. In fact the some of the least lonely elderly people I've met have been the ones that don't have children but are actively involved in their communities and have good links with friends and neighbours. The ones that have been completely enmeshed with their families tend to be the most lonely given children and other extended family members move away, work full time, have families of their own, die off etc and they don't have the extended network a community can bring.
Anyway I digress. Like most people OP, no one thinks your unreasonable to want to see your son. It's expectation and lack of flexibility and your own rigidity in your plans that is completely unrealistic.
Why does EVERY Friday have to be takeaway night with the same 15 people? You mention you can't go visit him as all 15 people would have to come to? Why on earth is that? The world won't stop turning if you miss a few of these takeaway nights. It all sounds completely stifling and suffocating. That all occasions have to include all these relatives, that there can't be any flexibility or change.
Your son is a young man in his 20s. It's absolutely normal and in fact healthy for him to want a life outside of his extended family however close he is to them.To want more from his life, have more experiences and have his life broadened than spending every Friday having a takeaway with the same people. I find it sad you don't count the time he spent with your on mother's day or any other occasions as meaningful.
Yes it's good to be close to extended family but how enmeshed you are and having no life or friends outside the same 15 people isn't healthy or normal. Taking BIL to work every day isn't normal or picking your nephew up from uni regularly isn't either. Are these family members are not independent or able to arrange travel themselves?
I find it sad you have no life outside this extended family. Your life isn't just going to exist with these same people indefinitely. The younger generation like your son and cousins will grow up and develop new relationships and form their own families and traditions. Older relatives will eventually die. Where does that leave you then? Definitely finding a hobby or job outside this bubble would be a healthy thing for you.