To arrive at the stadium for 12..
get the 09.54 from ponty you'll arrive at central for 10.23
jump on the Portsmouth harbour 10.30 off at temple meads 11.17
get on the Weston train 11.26 to parson street its only 6 minutes and 2 stops but it'll save you a mile and probably over 20 mins walking depending on your lads age.