Yes. He can sign up for Koodo Prepaid (SIM card costs $20 but comes with a $20 credit on his account) and ask to have his number ported from MTS. He'll need to provide proof of ownership of the MTS number (account number and any PIN and/or password) along with photo ID to verify that he is in fact who he says he is.
Koodo will then cancel his MTS account. If there are any credits on his MTS account, they'll be lost.
IMPORTANT: his current phone must be unlocked and compatible with Koodo in order to make the switch.
If you mean can he switch over his number, then the answer is yes he can! All he has to do is purchase a PREPAID sim card from a Koodo store/kiosk or online and then activate it here: https://activateprepaid.koodomobile.com/. Follow the steps to switch over his phone number and activate his prepaid account. Once you finish the process successfully, just place the sim card in the unlocked or Koodo phone and you're good to go! By transferring over his phone number, it automatically cancels his MTS account.
The sim card costs $20 but it comes with a $20 activation bonus so it pretty much evens out! Remember to buy any top-up amounts you need while you're at the store if you need extra.