My son started remicade at 18 as well. His GI checked levels at his first 8 week infusion (ie after loading doses) and found he did not have high enough levels and moved him to 6 weeks (he's been at six weeks for approx. 5 years). He check levels again last year and his serum levels were at either 12 or 20 (sorry, I remember they were higher than what is believed to be therapeutic but can't remember number exactly, although I'm leaning towards 20). When I asked about extending the cycle to 8 weeks, his GI said he was comfortable with my son's level as it allowed for a 'cushion' should an infusion be delayed. He said he had a number of patients at that level.
Also, has his dose changed at his 8 week infusion? ie remicade dosage is determined by weight (ie 5 mg per kg). If he was being given a certain total dose starting two years ago, he's probably gained weight since... has his total dose been adjusted? Here, the dosage is determined at the clinic at infusion but, perhaps, where your son receives his remicade, they wait for instructions to change the total dose?? You'd assume these things are being monitored but... never hurts to ask.
Also, agree with crohnsinct re the testing... if you're GI has not done full testing, it wouldn't be the first time GIs just 'assume' things are fine... :ybatty: You may have to insist on further testing. When was the last time your son had an MRE or scopes. For the first few years after my son's diagnosis, he was getting annual MREs and/or scopes (sometimes both in the same year).
And, on the chance that he's not absorbing enough nutrients, will he drink nutritional shakes? Since starting remicade, my son drinks one to two Boost shakes each day. It adds calories to his diet and increases his nutritional intake. But, keep in mind, the shakes should NOT replace meals... or you'll be no further ahead. See if he's willing to add them as snacks. The shakes won't fix any simmering inflammation but may help alleviate some of the symptoms while other things are being looked into...