How do I train my 6-month-old into sitting in his high chair while eating? He becomes cranky at the middle of his feed so I end up feeding him on my lap

Persistency is key! Train him to get used to being in a high chair. Sooner or later he should get accustomed to it. On the other hand, however, make sure that there's nothing wrong with the high chair, that it's not uncomfortable or hurting your baby.