Acidosis is a possibility if he has overdone it on the barley. he is still quite young for concentrates, especially if he is also eating (wet) grass... So yes baking soda may be a good idea, also if he is cold - I would put a coat onto him. Can you take his temp?
Barley can make them blow up quite easily, one of the reasons mine get oats.
My boy had acidosis as a young kid, and ended up at the vets for a couple of nights on a drip...