I have a few thoughts which may or may not help...one, there is an
alkaloid in lupines which can cause a type of poisoning, but the
symptoms are listlessness, anorexia, etc. so that's not it. Two, there
is a fungus which grows on lupines (primarily white and yellow
varieties, but not blue) and causes liver damage. The condition is
unknown here, since lupine isn't fed in this country, but it is common
in Australia and South Africa. Don't mean to scare you, might check his
gums for jaundice (yellow) just for fun. If they are, call your vet.
I don't think you're feeding enough of any of the feeds to cause a
protein problem, assuming that the pasture he's on isn't alfalfa
(lucerne) or clover. However, there is a specific protein that is found
only in oats that some horses are allergic to. It can cause the hives
you mention, also puffiness and filling, changes in behavior, etc. As
oats can vary greatly from batch to batch depending on your source, it's
not unheard of that he wouldn't react to it immediately.
Hope this helps a little.
Susan Evans
California State Polytechnic University