What Keeps Biting My Baby? Oz

Hi im having a problem with my 5month old... When he wakes most mornings i find just one bite on him (mainly around his head or on his legs) but it is a huge yellowy spot that is raised on the first day, day 2-3 it goes a nasty purply colour has a dry sharp top on it and then fades away ova anotha day or two... I have taken him to doc's, chemist, asked friends and generally am told it is a reaction to mozzie bites... It doesnt bother him at all, even if i touch it, and i have just been using a general antiseptic cream for stings... What worries me is if its just a mozzie why is he only getin one bite at a time, very rarely i find 2? Also i have actually caught mozzies stinging him when we were outside and the bite it left behind was a normal little spot. Could it be that because he is asleep and doesnt disturb the mozzie it bites for longer therefore leaving behind a nasty bite? Does anyone have any other possible insect/critter that could be causing this? He sleeps in a bassinette but regularly ends up in bed with me, and even then i find him with a bite the next day although i had no bites on me. He has mattress protectors always, his mattress is aired at least once a week for a full day in the sun, i've sprayed surface spray all around the bottoms and outside of the bassinette, his sheets are changed almost daily so what else can i do to find the lil critter? Any help will be greatly appreciated