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

Just like anything else, it may help if you let your baby get used to things you want him to do. Please do it with consistency so he can get the pattern. If you want him to stay put in his high chair while feeding, continue doing so even if he's cranky at times. This will eventually turn into a part of his feeding routine if done continuously.