You could also seek advice from your dentist, my daughter had this

How old is your son ?
It can be caused by a sharp or misaligned tooth for example but can also caused by deficiencies, in which case it may be good to have it looked at by the Gp

As an adult I’d say gargle with salt water often, but I’m not sure what to do for a baby.

I’ve had ulcers my whole life on and off, and so did number 2. Avoiding preservatives and rinsing daily with salt water are the best advice I have.

my son had a couple to but he lets me put ulcer cream on hope all is well

Did you see the GP or dentist? Bonjela soothes the pain

Oil pulling with unrefined organic cold pressed coconut oil helps my mum with her painful mouth ulcers. They’re usually gone in 2 days as opposed to 2 weeks.