I've been on Remicade and it has definitely helped my CD and I feel strongly that it also helps my back problems.
But, Humira is marketed for AS, RA, etc. So, it may be that there's something about it that would work better for your son.
I've wondered if I have AS...I've had chronic back pain for years. I've gone to physical therapy, gotten TENS therapy, massage therapy, and have been doing yoga regularly for months and still no relief with the dull, aching pain.
Can I ask, how did you find out about the HLA B27? Is it a blood test?
In my own research on AS, they say the best thing to do is exercise...ultimately you can't stop the fusion, but you want your back to fuse in a good position..so exercise and physical therapy techniques can help.
I wish you the best of luck with your son.