You have given me important information which does assist in advising you.I presume that your son did not burn his lip while drinking something hot or that he did not injure his lip playing rugby for example.The blister may be due to a herpes virus infection even if your son does not have a fever, but another possibility is an allergy.This allergy may be due to a toothpaste or to a food such as nuts for example.Another common allergy is to latex especially if he has been blowing up balloons recently.Change his brand of toothpaste to Elmex or similar type of toothpaste if you think that this may be the cause. Give him an antihistamine tablet such as Allecet 1 tablet at night for 4 or 5 days. If the swelling persists he should be seen by a doctor.
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