Canker sore???

My son is 16 months old. His gums beneath his bottom front teeth have appeared fairly red for the last few days and now I can see a sort of whitish spot below his teeth if I pull his lip down far enough. It seems to be bothering him a bit. He's been sleeping fine with tylenol, which we've been giving to him most nights because he's also getting a molar right now (great timing, right?) Is it a canker sore? Should I call his doctor? I've never had a canker sore myself so idk what to do lol. This is the best picture he let me take, ignore his crusty face, he just ate yogurt.

TIA!