8 is very young to be a full boarder, especially when parents are overseas and there are no close relatives/friends to whom they can go at weekends.
It is absolutely not necessary to start at prep school at 8 in order to go on to UK public school - not even to schools like Eton, Harrow or Winchester. Clever children start at these schools every year at 13 from schools abroad.
If you do want to go down the UK prep route, ten is early enough. Eleven is better. You can introduce sports, music, foreign languages etc at home before they start.
Boarding preps are struggling to fill places, especially for younger children as most UK parents no longer choose boarding at this age unless they absolutely have to. Schools mislead parents - especially overseas parents- as to the desirability of starting at 8 because they want to fill their places, not because it is in the best interests of the child. You can always take him out and start him again in two years. You will have lost nothing.
That said, if he has only just started, you should know that many young boarders have a wobble after the first exeat. The first couple of weeks are new and exciting, but when they go back after the first weekend away, the penny drops and they realise that this is now it. When you are abroad, it is hard to tell how bad it really is. He may have sobbed down the phone to you, and then cheered up again immediately afterwards - leaving you distressed while he has just moved on. The boarding staff should be able to tell you.
I do not know your family circumstances, so cannot comment on the decision to start him at eight. But you may like to ask yourself why you have chosen a path that very few British families now choose. Those that do choose full boarding at this age often have very unusual circumstances - parent/sibling ill, children living with grandparents, parent posted to somewhere unsuitable for children, member of royal family etc - .and even then they usually choose a school near family and friends so the child can go "home" at weekends. Some overseas families take this route but usually only to allow a child to learn English. As you are in the US I assume this is not a problem.
Lots of marketing goes on at this age group.