i would say so if he is 15 he will be fine its only a 4stroke they only go as fast as you want them too.
i know people that start off on cr/yz 80's and there fine you just have to get the hang of it.
I started on a 125 and i think they are a bit slow but thats just me, yeah a 140cc should be perfect . But maybe it went to slow cause it was a foxico? DUN DUN DUN !!!