Yes, you can use SIP phone with CCME, but the overall functionality will not good. Basically, cisco does not officially support non-cisco phones with CME. You will not have, or will have trouble enabling any feature like dual-line, call transfer, conference, etc.
I think the situation is not much different for CCM. Once something doesn't work with a non-cisco phones, you have no support from from Cisco.
I suggest you show him how the cisco phone works nicely in SCCP, with directories, etc.