Each ear does tend to give different readings by a point or two especially if the child has been lying on that side/ear or anything but shouldnt be a full degree different. Are you definitely inserting it the same depth and angle each time? As these digital ones really does need to use at a specific angle. Once when my son was ill and sleeping I didn't want to push it in too far and wake him and I got a reading of 38.4, about 3 minutes later he woke anyway and I fully inserted it and got a true reading of 39.5.
Try it in your own ear and see what you get, mine is also a braun thermoscan the blue and white one, I've never tried an under tongue one but I've used under armpit when he was a baby and I think they are quite reliable as our hospital uses that type too (only a bit more high tech than the ones you buy in shop). Does he feel hot to touch? If so I'd go by that rather than the under tongue one. My son was OK thanks it was a double ear infection at the time bk in November. When I'm using mine I push it in as far as I can but slowly under u feel as if the whole ear hole is "full" around it if that makes any sense.
Yes use ur cheek against his forehead or use lips, I always kiss his head as can never tell properly with hand either. I know how you feel, sick children make me very anxious too but as long as he's acting well, drinking/not lethargic etc I'm sure it will just be a standard virus. How old is he? Xx