UMass Amherst gives you the best of both worlds.
Due to the Five College Consortium, your child can take classes at Amherst College and other highly-selective LACs in the area. He can join any club at any other colleges.
My daughter decided on the other Amherst school as a CS major. AC is cheaper than UMA for our family. She can take graduate UMass CS classes as an undergrad at a small LAC and it’s all paid for.
You already know what reputation UMass has in CS. No LAC can touch it, nor can most T20 schools. If he picks UMass, he can also get an outstanding prestige liberal arts education if he desires, all at a budget price.