I took Tegretal and host of drugs and I think I know what he means.
The Tegretal won't give him a stabilizing effect the first day, but many people are drug sensitive and feel differently from the very beginning. That doesn't mean that the drug is already working as a mood stabilizer. It means he feels the foreign substance in his body. I always did feel something right away and usually, unfortunately, it was usually a bad feeling, although some drugs, such as Prozac, made me feel very good right away then stopped working.
It will probably be quite a long time before you know if this medication is a fit for your son. The only medication that worked great for me was Paroxotene. I've been on it now for probably twenty years and it still works really well for me, but I didn't feel the good effects of this medication for at least eight weeks and several adjustments. In fact, I was ready to give up on it too early and I'm really glad I didn't.