It does seem quite kiddyish from what i've heard, though I guess it would depend on your son and if he still likes the characters and all.
My sister had just that day turned 16 when we did the Keys to the kingdom tour and she didn't feel as if it was too old for her (16 is the minimum age for it) and she's not a very mature (now) 17 year old.
Maybe the Main street train one would be good, if you're set on a Mk tour? Not sure of the age limits on the ones in the other parks.