Just a note on the allergies, it is not proven that holding off prevents them. Infact there is some talk its' the opposite. all I know is my DD2 has severe allergies and we started her at 5 1/2 months, but DD1 has none and I started her on solids at 3 months 3 weeks. So definitley not a prob there.
We've just started DS at 5 months on the dot. I too was determined to hold him off he's also quite big, and I wanted to wait until 6 months, however he was sooooo interested in our food. As it turns out he is LOVING his food and I feel he needs it. I am only giving organic food and will not introduce dairy or wheat until 12-18 months. This is an allergy thing, as Iwant to do what I can to prevent him from allergies. However Im not convinced I can control it, and I think DD2 possibly got allergies from a flu injection I had whilst preg, nothing to do with solids intro etc. Also a bit of genetics (which is why I still worry DS might have some allergies).
I will also introduce meat when its' recommended, and just make sure it's organic. Alot of DD2's probs now come from the pesticides on fruit/veg, chicken hormones etc, so I feel just being organic is going to help DS precious little digestive system etc.
I also know someone who's holding off on the solids, and like RCC said, he's kind of like a baby, not developed iykwim.
Hope that helps a bit xo