###Yes he can save himself. Yes, he will lose his casting of Summon Monster V.

Summon Monster has a [casting time](http://www.d20pfsrd.com/magic#TOC-Casting-Time) of 1 round, and the spellcaster is required to concentrate on the spell for that duration (emphasis mine): 

> When you begin a spell that takes 1 round or longer to cast, **you must continue the concentration from the current round to just before your turn in the next round (at least)**. If you lose concentration before the casting is complete, you lose the spell.

[Duration](http://www.d20pfsrd.com/magic#TOC-Duration) states:

> You can't cast a spell while concentrating on another one. Some spells last for a short time after you cease concentrating.

There is no exception for immediate action spells in there, nor anywhere else, meaning that you will have to drop concentration to cast another spell. The rules don't state any sort of action for dropping concentration voluntarily, but dropping it involuntarily (by failing a concentration check) is not-an-action, so I suggest doing with that.