Your son sounds like a joy and a handful! The story you tell makes me wonder a couple of things: Have you checked his gums to see whether he is teething? Sometimes, especially when a child is in a growth spurt, he'll eat anythng no matter how much pain it costs him! Also, have you considered cooking or preparing food for him that he CAN feed himself? One of my friends would steam vegetables--whole carrots, parsnips, sweet potatoes (one of my favorites for my three children), anything large enough for a little one to hold onto and gnaw upon. I often just put our food onto our children's plates, cut up, ready to eat with their fingers, which they happily did. We have a very humorous photo of a 7-month-old Caitlin smiling through the burger and beans she had just eaten --and slathered all over her face! It was well worth the bath that resulted... Could your son's sitting down after such an active time be the signal to his little body for him to become fatigued? He could be simply exhausted ... I mean, when I'm really tired, I'll cry through my meal, too!
As to your second problem, my son was the same way and I just learned to move quite quickly. Additionally, he could sometimes be distracted by certain toys, or a treat of some sort, long enough for me to speed-change him! He also loved hearing me sing, and that could calm him for awhile, too. You'll find something that works, and this phase will pass, too...