Welcome to the forum crohn'sparent, check out the parents of IBD section when you get a chance there are quite a few of us with similar aged sons. Mine will be 14 in just a couple months.
Yes there can be scary side effects with all these drugs, but same can be said with under-treated Crohn's and ongoing inflammation can have some serious side effects as well. As you will find and read, we all wish as parents we had a crystal ball and could know what the right choice to make as a parent is. When it comes right down to it we want them feeling well, pain free and growing and developing like their peers.
There have been many that for them 6MP was a miracle drug and gave them back their child for others it has not worked. It does take a while to be effective several months in fact and dosage may need to be adjusted within that time. Unfortunately we don't know which meds or combination will work until we have tried them.
My son was on Imuran/Azathioprine for several years with no side effects, which is a similar class drug as 6MP. They do regular blood work to check for theraputic levels and liver toxicity. He also had a metabolite (or TMPT) test done before hand to see if he could metabolize it before he started taking it.
We had a hard time reaching theraputic levels apparently he metabolized it a little too well and had to add allipurinol to increase the potency which several other parents have had to do as well.
My husband has been on it for the last 8 years and was on Pentasa only prior to that and had a lot of issues and symptoms. He has had very few problems since he started Imuran and no side effects.
There is a thread/club in the parents section that talks about Imuran/6MP here
http://www.crohnsforum.com/showthread.php?t=44911 (OOPS I see Jennifer already posted this.)
I hope you find the support and answers to your questions that I have from this site. And your son is feeling great soon