Kids will adapt...daughter at 7 yrs old riding a 130 at 9 jumped to a 148 stiffy freeride...(I put on 20mm risers so that she could edge better....the result is that she developed some great edging skills)kicked her butt the first year, second year was handling it and third year throwing it around like a park board. At age 12 then went up to a 155 for blasting groomers and pow, at age 14 went to a 144 park board and at 15 switched to thug skiing...fatty twintips with AT bindings...and Tuesday was her first trip ripping some out of bounds with 25ft cliff drops. Later today she's back on one of her boards for the third time in about 40 days out this season. Btw, she's been 5'4" and 125# since being 13 years old.
Point being kids will suprise you...I'd think twice about detuning....he doesn't know any better and if he has a passion, he'll figure it out...a 144 doesn't sound out of the realm.