Tourettes is VERY cyclical and has known triggers though it can vary by individual. Known triggers are stress, tired, bored, excited (even good excited). Most people, like 80% almost entirely outgrow it after puberty. It is very rare for it to worsen and overwhelmingly it lessens in adulthood. Whether this is for immunological reasons (some people think this is caused by infectious triggers in genetically sensitive individuals) or neurological maturity is unknown. Hope this helps.
I'm not a doctor, but I do have Tourettes. It's technically a relatively mild case, but It has really bothered me since I was a little kid. My understanding is that it's not uncommon for symptoms to tapper-off a bit with age. Mine have noticably diminished as I've gotten older (I'm 42 and male), but they do flare-up now and then - usually under stress - but not to the extent they did in my teens and twenties.
I've never heard of tics being "seasonal", but again, I'm not an expert.
Here's a link to a useful site just in case your son hasn't already found it. http://www.tsa-usa.org/