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

Babies, like most anyone, don't like change. To get your son accustomed to sitting in a high chair, I'd recommend the same strategy you've used to introduce anything to him at this point. For my daughter, the best way to get her acclimated to anything new was to just stay patient. He'll eventually ease into the concept of eating in his high chair if you just keep at it.