i totally agree with the allergy theories. 100%
in the UK there is much discussion about the increase in allergic diseases in children, asthma, excema etc and adult onset allergic diseases such as hayfever and coeliac disease. I don't know if anyone has done any real research into this, but i wonder if the increase over the last 20 or so years has a lot to do with the fact that 20-30 years ago it was recommended to wean babies at 4 months or even earlier. maybe this increase in allergic diseases is all to do with early weaning? i don't know but it would make sense to me!
besides if you wait until 6 months, you can try a more varied diet more quickly, AND babies can get involved and participate more themselves, which has to be good!
maybe you should suggest to DH that one of the biggest indicators of a baby being ready to wean is the loss of the tongue thrusting reflex. so try putting a plastic spoon in his mouth and if he pushes it out again, he's not ready. job done, DH will have to wait