A painless hard round lump under the tongue after a hot tea burn is most often a blocked salivary duct or a mucocele/salivary stone, sometimes just a healed scar nodule. Painless does not mean ignore it - any oral lump lasting beyond 2-3 weeks must be examined directly, especially if he uses tobacco or gutkha.
Next Steps
Get it examined in person this week - a physician or dental/ENT surgeon; an ultrasound of the floor of mouth settles it in most cases. Warm salt-water rinses, avoid poking it with the tongue, stop very hot food. If it grows, becomes hard-fixed, bleeds or ulcerates, see a doctor urgently. Happy to review it on Practo consult or at Synergy Multispeciality Hospital, Miraj.