That or this?

My boyfriend always says "that's good" when he eats something delicious... It sounds weird to my ears. What would you say? Is he right?

According to your context, his spoken English is poor if he uses "that" instead of "this" when you are there when he makes the remark to the food he is eating.  However, if he was telling you something he ate while you were not present, "that" is the correct form.  People usually speaks the first thing in the mind and we make mistakes all the time.  The most important thing is to understand what was meant.  It is different from writing.  Error will still glare back at you unless they are corrected while spoken words are gone and forgotten.