There are many good preschools in the US, and especially in California. Don't worry about the English. As he is so young, it will not take him a lot of time to assimilate into the classes. I don't think it is necessary to repeat a year.
关于费用,美国公立学校的幼儿园不一定是免费的。加州的公立学校幼儿园可能是免费的。你查查。如果不是,索性找个好一点的私立幼儿园。 就是班级比较小,幼儿园老师很有爱心。建议你考察下面一个学校的幼儿园:
Waldorf School of the Peninsula
Mountain View Campus
180 N. Rengstorff Ave.
Mountain View, CA 94043