Just book under your name. Usually the name can be changed on the bill on checkout. If your colleague doesn't check in together with you, tell him that he has to ask for a reservation in your name -- and to mention that there should be a second room reserved under the same name.
HTB.
The easiest approach for all involved would seem to simply be to make the booking in the other persons name and be done with it . Less effort to make 2 bookings than have to get things changed later and easier for other person if he arrives at a different time