If Dad comes home early tomorrow night, he can help you with that.

I made up these sentences so there's no source.

A: I have trouble doing my math homework.

B1: If Dad comes home early tomorrow night, he can help you with that.

B2: If Dad comes home early tomorrow, he’ll be able to help you with that.

I think the main difference is that B1 sounds like B is suggesting Dad as an available option for A to consider asking for help from while B2 sounds more straightforward: Dad will have the ability to help A given that he comes home early tomorrow.

I am not sure if I've distinguish between them correctly, but I know there's a difference. Please share with me your opinions.

I'd say B is more natural. B1 is general while B2 is specific.

MichaelLu2000 said:

while B2 sounds more straightforward: Dad will have the ability to help

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I agree.

MichaelLu2000 said:

am not sure if I've distinguished between them correctly,

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Because "can" expresses ability or possibility, using the standard ("will"+Vbase) "will be able to" is preferred here.

Note that "can" does work in some conditional-1 contexts. For example:

If you promise to be polite, you can come to the party.