As long as he is growing and staying active, then you have nothing to worry about. Failure to Thrive is where they stop growing, lose weight and are not as active and that will be what you have to watch for. It sounds like maybe you need to get some sipper cups for him. He very well could be wanting to be like big brother. Mine all stopped letting me feed them very early too, because I didnt feed their older sibs. They are very independent, even if I dont want them to be!! lol They want to imitate the older sibs, my 2 yr old is positive that she can do ANYTHING that my 7 yr old can do, even when she was 6 months old. She even started talking at an early age to keep up with my son. She will be 3 in August and has the vocabulary of a 4 or 5 yr old. Its amazing how much these younger kids soak up from the older ones. My baby is going to be my earliest walking I think, she is 10 months and starting to pull herself up. Now before you say, big deal, let me add that my other 2 were way over a year before they even tried, my son was 15 months, and my 2 yr old was almost 18 months. The baby wants to be with her older sibs so badly that she is trying very hard to keep up. lol So try a sippy cup, or if you son uses a regular cup, try that. Watch that he doesnt fall into failure to thrive mode. And Good Luck!