10 week old baby length and weight both 85th percentile. Head circumference 95th percentile. Does this mean he has a big head in proportion to body?

Not really. I don't consider that difference worth anything more than noting. Big babies are big all over and this falls within the normal variation of such measurements. If the kid was 50% weight/length and 95% head, I would just measure the parents head to see where baby inherited it from. There are other issues to consider with a big head, but not from the reported info.