Hello, as per your description ur son might be suffering from soft tissue infection
of mucosa(most common-primary herpetic gingivostomatitis
), its common in 3-5yr age group children.. .. give him proper rest.. and regulate the body temperature by giving NSAID- like paracetamol
..This disease usually go in 7 days by body immune process only symptomatic treatment is given.. If it persists later then that investigations are performed.. our body defense mechanism is sufficient to cope up with disease if we are in healthy stage.. The pain in lesions might had been causing him difficulty in eating and swallowing.. so use a anaesthetic oral gel 15 mins prior to his eating or drinking schedule let him use antiseptic mouthwashes after eating to prevent secondary bacterial infection in the erosive lesions meanwhile..