I've been through it. My kids didn't want to sit up in their high chairs; they would always crawl and try to stand up and get out. Well, there are some reasons. He may feel uncomfortable sitting in his chair. If he's always acting up in the chair, think about purchasing a better one or feeding him on your lap. The second reason is that he might not be hungry, and you're trying to make a feeding schedule strict. He might hate these times when you do that. Try not to force your kid to eat. Just read some articles about signals which indicate that babies are hungry. The third, most unusual reason is that he might feel lonely. He might want to communicate with you. My kids sit in their high chairs more readily when all of our family members have dinner together. Besides, your child can be bored!