Present or future... which one should be used here?

Question

I am in doubt about how to refer to a situation that may occur in the future, but based on factors that already exist in the present. Should I use the verb in the future or in the present? I'm using as an example the sentences below that I created myself. Which one (or perhaps more than one) would be correct?

Mother talking to her son's future teacher:

"And if you notice that he's having a hard time making friends, that's because he's a very shy boy."

"And if you notice he's having a hard time making friends, that'll be because he's a very shy boy."

"And if you notice he's having a hard time making friends, it will be because he's a very shy boy."

Answer 1

Both present and future are possible.

I would typically opt for the present.

However, there is a nuance added by the future, which is that the mother isn't convinced that her son's shyness will cause him to have a hard time making friends. In the present, it leans more towards a fait accompli.

This is a pretty faint nuance, though, and the present would be a safe choice either way.

Answer 2

All your forms are acceptable, but there is nothing "future" about any of them.

There is a long-standing myth that expressions using the modal will are a "future tense".

It is true that such expressions often refer to future time, but not always (and they are often not required to refer to future time).

All modals have an epistemic reading as well as their primary deontic one - a sense to do with the nature, quality, and source of our knowledge. Often they can be paraphrased as "I/you can assume/deduce/conclude that", and that is what is going on here.

"That'll be because he's a very shy boy" means something like "that will confirm to you that he's a very shy boy". It has a rather different meaning from the directly stated (non-modal) "that is because he's a very shy boy".