SE v. ME
At least two 141 programs in which I have instructed, ERAU and FlightSafety, have students first obtain their Private multis so they can log PIC multi while training for their Commercial and Instrument. They get both in the multi and go back to simple singles for their Commercial SE. It's a great method because they can maximize their multi bang for the buck. Both 141 programs are set up accordingly.
In this case, the student is under Part 61 and has most of his time for his Commercial. He could fly the multi around for thirty hours until it is time for him to go for his Commercial, but it strikes me as a waste of money because it would all be dual multi time when multi PIC is what matters. You could get him his Private Multi in ten hours so he could log the rest as PIC, but he would have to shell out an examiner's fee, which he would have to pay again for his Commercial. Although I love the idea of getting initial Commercial certification in a multi, paying two examiner's fees so close together does not seem worth it, in my $0.02 opinion.
Having said all this, he's probably best off at this point to finish his Commercial in a single. You can then get him his Commercial-Multi add-on in ten hours. Of course, he won't have much multi or multi PIC, but plenty of people have added their multis after getting their Commercial and have gone on to success.
One final idea might be that after he earns his multi that he continues in it to earn his initial CFI as an MEI. That would give him some multi PIC. His CFI-A and CFI-I would be add-ons that can be done in a simple single. There are people who earn both MEI and CFI-I as initial instructor certifications and then add the CFI-A in a single.
Hope these thoughts help. Good luck with however you and your student proceed.