Dear Jaev,
Lamictal can cause tremor, and increase unwanted movement, like tremors, spasms, tics, etc.
You could try this link, and search down the page for "tremor".
http://www.rxlist.com/cgi/generic/lamotrigine_ad.htm
In situations with med problems that are not dangerous or acute, we have asked the neuro about pulling back to a previous dose and see if the side effect improves, (don't know if your son's seizures can handle that without adding on other meds, etc....). I would be really leary too of increasing it.
This side effect was very severe for my child, who had to be sedated while the lamictal was weaned very quickly. His situation is different because he has previous history that made this problem more likely, but I would agree with you about being nervous. In one increase during the titration we went from okay, to really bad problems.