To stay for more than 90 days he would want to enter under a visa, not VWP - he cannot change his status under the VWP so he would have to leave within its 90 day maximum duration. He would need to obtain status that permits the extended visit. Even on a B2 he can expect a maximum initial stay of six months - but prior to the expiration of that stay (and I suggest early) he can file Form I-539 (Application to Extend/Change Non-immigrant Status) asking for an extension. He should expect that the USCIS will want to be convinced that he can support himself throughout his stay - that is, that he won't be (unlawfully) seeking employment.

