Does it almost look like a welt? My dd (and myself) get wicked reactions to mosquito bites and other bug bites. One time dd had a bite on her calf, and half of her leg looked swollen! But it was just the bug bite and the way they make those welts on our bodies. It happens to me too.
I'd trust your instincts and keep an eye on him. Is he running a fever or anything? If your instincts tell you to get it looked at, I would, just for your peace of mind. I didn't take my dd in because I get the same reaction (and so does my brother) so I know our "sweet meat" kind of runs in the family
I usually make a little paste with some warm water and baking soda and put it on the welt if I can... it seems to help a little bit. I've had to go the hydrocortisone and benadryl route with mine because I have had several at one time that were bad... but that is up to you if you want to try those things. If you really feel it's a bug bite maybe someone has some more natural remedies to help speed the healing.
ETA: If it is more like a red "streak" then a swollen welt, I would definitely take him in to get it checked. As I mentioned earlier I have always reacted to bug bites, one time I got bitten in the arm.. and what started out as a routine bite turned into a staph infection. Thank goodness it was caught early... but I had this red streak running down my arm from the site of the bite and it wasn't pretty. I also don't remember any other adverse effects, just having that red streak, but it did turn out to be serious. So go with your instincts and see how it looks as you check on him at night.