Your right he could put a full size 2 pole breaker in that panel but that would take 4 1 inch spaces which he doesn't seem to have. His best choice might be to increase the ampacity of his service and install a 40 by 60 space 200 panel near the existing one. By using the required exterior disconnect as Service Equipment he could run the a 200 ampere feeder to the new panel and use a 100 ampere breaker to supply a feeder to the 100 ampere panel. If GE makes a 40 by 60 circuit panel he would want to use one to save the cost of all new breakers. He could then move all of the double pole breakers, no matter what loads they supply, into the full size breaker only portion of the new panel and move the 120 volt only breakers into the portion of the panel that excepts 1/2 inch type as his time permits. Most of the full size only breaker slots would still be open for future loads.
I have seen assertions on the Electrician talk forums, which are only open to use by professional electricians, that he would need to comply with all of the Arc Fault Circuit and Ground Fault Circuit Interrupter requirements while others claim with, equal certainty that is not required. I suspect it is required because a service ampacity increase is not a one for one replacement. I haven't taken the time to research that yet. If someone already knows the answer then please chime in.
Tom Horne