We have not had many problems with helmets, but we have rarely used helmets in the trailer. So helmets don't really come into play for us until they come out of the trailer (onto their own bike, the TAB or the tandem) which is about 3+.
At 20 months he is likely not talking very well yet and reasoning is also probably difficult. Any way that you could maneuver him into being excited to pick out his own helmet? You would get "buy in" that way. It also could be a fit issue. Something to consider, is it the helmet he does not like or the position of his head in the trailer with the helmet on? The seat backs along with the protrusion on the back of the helmet can force their head down. A neck pillow or towel around the neck can help get them into a more comfortable position.
The helmet should not be an issue too much longer. He should be more compliant to rules in the coming months.