OMG, OMG, this question made me flinch! As the mother of five kids, NONE of whom were under 9lbs at birth (smallest 9lbs even, largest 10lbs 8oz), i have gone thru this EXACT same thing! ALL of my boys were - at this age - off the charts for head circ / height / weight (pick two or three). My oldest daughter was over %100 for height at one point but under %50 for weight.
Mine self weaned from breastmilk between 6 months and 1 year. I fed them meat, eggs, veggies, and a small amount of fresh fruit and NO COW's MILK. Their doctor predicted horrible things.
End result: these kids (now ages 4 to 9) are all now slender, active, well-within-range children. They are not Paleo but are low-carb, low-processed-crap.
My advice would be to not feed the child any milk at all. A 15 m/o can eat exactly what mom and dad eat chopped into small bits. If your friend insists on milk I'd recommend full fat whole milk - organic if possible.
I'd also recommend that your friend just not bloody worry about it. Seriously. Kids wax and wane all over the 'official' charts. Unless the kid is becoming overweight (and it doesn't sound like it), then I'd just blow it off.
To give you some perspective: my oldest, whose head circ and height measured off the charts for his first two years is now a SHORT-ish 4th grader but he's just been awarded a certificate for making the highest score possible on his standardised reading test.
Tell your friend that that big head is there for a reason, LOL. It probably houses a big brain and that brain needs FAT! ;)
Best of luck to your friend and his baby!