Yes, my work is Bangkok based, so all our extensions are done at CW (annoying!).
Last time (5 years ago) we did the transfer at the same time as the Extension and it was a nightmare. We had to get a queue ticket for N2 for the transfer, and 3 separate queue tickets for N1 for the extensions. The queue for N1 went quicker than N2 which meant we couldn't do the extensions and had to get new queue numbers. I vowed to always do a transfer separately after that!
In any case we fly to the UK for a holiday in April, and my thoughts are that it will be easier if he has a new passport rather than one expiring in 5 months from when we leave. I guess, he could travel with both the old (cancelled) passport showing the Extension and his new passport, if I want to put off the CW visit?