There are no set guidelines for exactly how many times or for how long he can leave Canada while his sponsorship is in progress. He just needs to be living in Canada. It might be a good idea for him when he gets to Canada to file the sponsorship to call CIC and ask. If it was me, I would not risk leaving for more than a couple of weeks and I would have somebody going over to check my mail and answering machine every day in case they try to get in touch. It's also better that he stays until they are done with the part to approve him as your sponsor (38 days currently).
Let me understand, just for 38 days he must be on Canada?
Are there a time to apply for the sponshorship? I mean, if he stay here couple of months and then he go back to Canada, he can start the sponsorhip process until the day he arrive to Canada?