Thanks for posting on HCM,
The cause of the lump will depend mostly on the characteristic of the lump. It could be a secondary effect of either of the medications you are taking (which I doubt given that it is only an isolated lump on his jaw) or infection (especially as he is already having a cold/cough
Try giving it a few days to clear off on its own and if it does not, do consider seeing his pediatrician
for proper evaluation.