Have money? Want to live away from people who don't have money? Holden it is.

They even talked the WRTA into removing the Worcester-Holden bus line, so you won't have to look at those nasty, filthy poors who don't even own cars.

Overall Massachusetts is a great place for Asian Americans, in my opinion. Shrewsbury Public school has a 33.6% Asian enrollment rate. Hopkinton 26.4%. Grafton 13%. Unfortunately Holden doesn't publish its demographic numbers. I would definitely look at the enrollment figures over the town demographics because there are a lot of old people in these towns who have no children in the schools.

Since house prices won't really matter, I'd pick Shrewsbury. It's a great place and very close to Abbvie.

Worcester proper has a decent Vietnamese community. There are specific businesses that advertise in Vietnamese.

Holden is white, but very nice.

Holden is Wachusett regional school, which is a district made up of Holden, Sterling, Paxton, Rutland, and I think another town. Holden is the nicest and most diverse of them - but all are very good towns.

Massachusetts is very different than orange county! From Holden you can be in 3 different states in half an hour. Distance is different. Almost anywhere in central mass is an easy commute to abbvie

With a child on the spectrum, Holden is a good bet. He'll get better services through the Wachusett school district than through Worcester. Worcester schools are fine but if I had an impacted child I'd prefer to be in Wachusett. Wachusett might have more opportunities for gifted students, but, the kid is 4. Talented in 'music, computers, STEM, and maps/geography' at 4 years old sounds a lot more like a parent over-embellishing their first born than reality. Either way, Holden is a good bet.

I am not asian american but Holden is fairly affluent and welcoming. Most of MA is, really. As others said, Worcester has a decently sized Vietnamese population and Worcester borders Holden; might be able to find some community connections that way. Again, with a child on the spectrum I would opt for Holden, but depending on where you live in Holden everything in Worcester is a short drive.

Holden is nice. I grew up on the Worcester-Holden line. Asians will feel at home in Worcester or the immediate surrounding towns.

Move to worcester, more diverse and worcester public has great response to kids needing extra services.

Let me preface this by saying that I'm on the Worcester School Committee, so I am not going to pretend to be unbiased.
It may be useful for your cousin to know (as you're discovering) that Massachusetts school districts are much, much smaller than California districts and much, much more segregated by district than California districts. Our suburbs tend to be much whiter; much of our diversity (not all of it, but much of it) is in our cities.
Because of the relative size, there are programs for students (autism or otherwise) in some of the cities that smaller districts don't have the capacity to run. That is not universal, but note that many smaller districts will send students with special needs (depending on the level of need) to schools that are not district-run schools.
Worcester Public Schools does run a comprehensive program for our students who have autism. I have not experienced that as a parent; I do know of parents who speak of it highly.
If your cousin is intending to choose where to live based on the ability of the public school system to serve his son, I would strongly urge him to get in touch with families of students with similar needs within that district.