i have ideas, and tricks, because i have awful allergies, but nothing perfect.
you can try given him B6 and B12. that helps some people
When he goes outside, he could wear a dust mask. uncomfortable, and people will laugh, but it will help.
there are several types for sale here
http://www.tasco-safety.com/respirator-dust-mask.html
Frequent showers and washing his hair a lot will help.
What plants are around your house?????
particularly any that are year round in Texas?
You could get rid of those, but until he has allergy tests, i don't know how you would know what plants you could use instead.
Aside from that, moving to a place with very different plants MIGHT help. no guarantees.