There's not necessarily any link between croup and asthma, but there is between allergy-type disorders, like food allergies and eczema. Croup is caused by an infection of the voice box, whereas asthma isn't really caused by an infection, but more by an over-sensitive immune reaction in the lining of the airways (although, that said, it can often be exacerbated by an infection).

Given his history, I'd say your little man is on the road to being asthmatic, but time will tell. At this age, even non-asthmatic children will develop asthma-like symptoms (wheezing, shortness of breath) when they have respiratory tract infections.