Welcome to Healthcare magic forum.
Your son most probably has symptoms of a mild upper respiratory tract
infection with inflammation of oral mucosa
(ulceration/pustule).I assume his dental hygiene is adequate and there are no teething
issues like carries, gingivitis
etc which can cause irritative lesions on the tongue.
At present the condition doesn't seem worrisome . As majority of such infections in children are viral in origin which are self resolving within a weekor two, you may wait and watch for it's self resolution.
In some cases if the lesion increases in size , worsening pain with fever, or discharges pus , it's better to give a course of antibiotics for preventing any complications and for faster relief.
Meanwhile simple warm saline or betadene mouth washes or gargles done thrice daily may promote faster healing and soothing relief.
Some anaesthetic mouth ulcer
gels( like xytee , stomela etc) may provide temporary relief from the pain however they are best avoided in young children.
If the problem persists or he develops any new symptoms like fever, lymph
node swelling, oral bleeding, dysphagia (pain on swallowing)do consult his pediatrician
for a thorough evaluation, confirmation of diagnosis and appropriate antibiotic therapy.
Wishing him a speedy recovery.
Hope that answers your query