my ds had a dairy and soy intolerance as an infant (bloody and mucusy poos) and he did great on alimentum - I think that even though the proteins are derived from dairy, they're broken down so many kids can tolerate it.
But obviously not all cant.
Good idea to have a back-up plan though - starting with elecare could probably work if he'd accept it (it's really really GROSS!) - if he really reacts strongly to dairy and soy I'd probably do the same thing.
I don't know why our dd didn't do well on it - we had to stop bf'ing abruptly and she needed to be fed through an ng tube with the elecare. She would vomit it all up violently shortly after almost every feeding. It could have either been irritation from the tube (although eventually I was able to try breast milk again and she tolerated that just fine through the tube) or the fact that she was reacting to the formula itself, which is not really supposed to happen.
I can only imagine that if a child has a sensitivity to corn that they could react to elecare - the first ingredient is corn syrup solids (different from HFCS). But who am I to question (at least according to the doctors;) - who also, by the way, assured me it was okay because it was made of "corn starch". whatever. I obviously have some baggage there;) Maybe I'm not the best person to be giving you advice on elecare, lol!
But seriously, it's supposed to be the least 'reactive' thing you can get as far as formula is concerned. And if you just want to have it on hand in case you need something to supplement your own milk supply, cost shouldn't be as much as a factor. The biggest thing I can see you having an issue with is getting him to accept it.