TalkTalk only charges £5 per month to use its email service for people who aren't customers of TalkTalk. Those who get their broadband service from TalkTalk get the email service for free.
Gmail is free to use irrespective of which broadband service and/or mobile phone provider one uses. Your BIL simply needs to get himself a Gmail address and configure the mail app on his phone to use it. (Better still, he should switch to using Gmail's own app).
If your brother wants emails sent to his TalkTalk address to be forwarded to his Gmail one, he should try using the 'mail collector' facility offered by Gmail.
(Just a personal note here: I much prefer GMX to Gmail. I find it far more intuitive to use and their phone app is excellent:
https://www.gmx.co.uk/ )