He is also crying a lot and seems unwell. No temp or at least not an obvious one. He also has a cough though that might be unrelated.
The red patch on the tongue is on the side. It almost looks like a burn or like he's bitten himself but I don't see how those things can have occurred as he only has front bottom teeth not near the site patch.
It is very difficult to see the sore bit as he wont open his mouth on request! I am trying to get a pic.
I am worried. Especially about the relentless crying