Your son might want to get himself on a diet.
http://static.e-publishing.af.mil/production/1/af_sg/publication/afi48-123/afi48-123.pdf
According to AFI48-123 (page 50), for orientation and incentive flights in ejection seat aircraft the weight limits are:
201 for B-52 and U-2
211 for T-38, F-16, F-15, F-22, A-10, B-1, B-2
245 for T-6
The specific standards are in AETC Form 35 and match the same weights for training standards. T-38 standard is actually 240 lbs.... but once they leave the T-38, if they can't meet the weight standard for other ejection seat aircraft, they will be limited to non-ejection seat. Since T-38 is a universally assignable trainer, he could be sent to any heavy. There are weight standards waivers, but medical will put him on a weight loss program first and evaluate for any underlying conditions that could cause excess weight. He doesn't want that extra scrutiny, any chance to be DQ'ed is very very bad. He was fine to get an FCI for T-6s since he meets that standard, but he's pretty much limited to non-ejection seat aircraft right now.
Also, you said 18% body fat. I'm assuming that wasn't a typo since 8% would mean he's over 7 ft. At 18% body fat, he's at the upper limit of 'normal' and hitting overweight. In a BMI chart, he would be classified as overweight approaching obese. I understand body type and he's probably very built, but he has a lot of fat he can lose with some cardio. As Pima suggested, he won't have time to go 2 hours a day to the gym so he will lose some weight there as well.
I'm not sure why the flight docs aren't advising him well, so it seems. The standards are pretty well-known for them and easy to find. He qualifies for waivers most likely, but why risk it? And he has to remain in weight after UPT as well.
If I was mentoring him, I would be telling him its time to run and cut weight. Getting a fighter, in his current state, will be challenging from the medical side.