Here is my take on this:
This greatly depends how much and what part of the tongue was removed. Perhaps someone with more experience can pipe in, especially those who had reconstruction surgery as well.
People learn to compensate, depending on the extent of glossectomy a speech pathologist might help. It also depends what language you speak, I have issues with a sharp ss in english, a sound that is not much that much used in my "mother tongue" (swiss-german) so this also partially depends on the language (at least in my case). A pity that I get to speak this only rarely these days. I have been teaching (in english) since spring and speech has not been an issue as far as I know (on the other hand this WAS Pchem....). Despite this and other things one might complain about, these "inconveniences" sure beat the alternative.
I suggest you talk openly with him; the treatment is rough and now is not the time to fret about issues that he may or may not have a year down the road.

Best

Markus