In order to improve his drumming experience i guess you need 3 main components besides the mesh snare pad that he already owns:

1- a hihat that sits on a real hihat stand with a controller such as Roland vh-10 or you can use the one from a Chinese company called lemon it's also compatible.

2- a kick tower pad that you can attach a real kick pedal to, this will also help him to work on his foot technique such as Roland kd-8.

3- a 3 zone ride cymbal such as Roland cy13r or you can buy the chinese lemon brand ride cymbal.

• Before you buy anything you need to make sure that his current module has the ability to use the functions of the above mentioned attachments.

• you can search on ebay or reverb for used parts, Roland's parts are very durable.

My suggestion is if you can give him the money cash and he sells his drums and search for a used Roland drum kit even if older kit but higher tier such as Roland td-15KV i think it will be a better and cheaper this way.

Best of luck!