Agree with chapsmum. So long as your ds is growing on the same centile (in my dd's case, she was below the 0.4th!), I don't see any alarm bells with weight at all. Those charts are just a guideline, hardly the gospel. FWIW, I think they are evil. If your ds is otherwise developing nicely, generally happy and active, what's to worry?
BTW, 6oz feeds 5X a day is A LOT, at least by my book. My dd at that age was breast-fed so I could not tell, but I am almost certain she did not take in half of that amount.
I was also asked to give solids to dd at an early age by all health professionals. Despite our best efforts, dd never really took to solids until she was more than 9 months, and even then it was a slow slow slog.
At 6 months, your ds' digestive system is so immature anyway he will absorb more calories from milk than food. So long as your ds packs away the milk bottles, again, I wouldn't worry about the fact he is not taking to solids at 6 months.
For solids, you don't have to use a spoon. Just put a bit on the tip of your finger. Different children take to solids at different times, but always when they are ready. Easier said than doe, but try not to compare with your nephew! 