Hi @neil53 I think the answer depends on a lot of factors, not least how geographically close to each other they are. First, do you know if there is any particular reason why your son does not spend time with his disabled brother? Is there conflict or disagreement between them? Sometimes siblings just aren't as close when they grow up. Second, do they want to spend time with each other? Third, assuming yes, are there things that they have in common? It may be that a prompt from Dad is what's needed – pointing out to your older son something that his brother would be interested in that they could do together. How involved you should become really depends on how happy each of your sons are with the current situation. If they are content with it, then as adults it's their business, however regrettable it might seem that they aren't closer.
All the best to you all.
I work as an Information & Advice Worker for Scope and parents very often express concern about their childrens' relationships. I have always found the sibs website a really useful source of information about the dynamics of sibling relationships when one of the siblings has a disability.
Have a look at their website - they have separate sections for young & adult siblings:
I hope you find it useful - let us know how you get on