Morning, I've not done it from Bristol but I love planning stuff out like this and may help others out aswell
If you use a website called Split My Fair and get the train from Temple Mead's to a train station called The Hawthorns it will cost in total £30.88 for the return ticket. It's then approx a 6 minute walk from The Hawthorns Train Station to the stadium according to Google Maps. (0.3 miles away)
He would have to change at Birmingham New Street and walk to Birmingham Snow Hill which according to Google Maps is a 5 minute walk (0.3 miles away)
Train time i've found which is cheapest is the 08:42 out of Temple Mead's arriving at The Hawthorns for 12:27
And the return leaving at 17:42 arriving back in Bristol for 20:30
https://www.splitmyfare.co.uk/