If it helps you at all Nickikate, I've got a three month old son, about the same weight, and am a first timer. I'm breastfeeding, so I can't be sure how much milk he's actually consuming, but I think I must be giving him about 4-5oz per feed, and he feeds 7 times every 24 hours (including 11pm and 3am), so he is probably getting about the same as your little one.
I can understand the temptation to wean - as I sit with my breakfast, my son watches my every move and dribbles prolifically, hands twitching and a cute smile on his lips. However, having read up, it is not a good idea to wean too early. As other mumsnetters have mentioned, early weaning can lead to poor health in later life, e.g. obesity, diabetes etc. Some research suggests that babies' intestines & some of their other organs (specifically their kidneys) are not ready to digest solids until about 6 months. As NotQuiteCockney has said, adding baby rice to a bottle is supposed to be a choking risk.
I can understand the temptation to wean early - not only do they look as if they'd love to tuck into what you've got, but it might help them to sleep through. Personally, I've decided to try and hold out till about 5 months before weaning, which is still deemed a little early, but I wouldn't even consider giving him anything other than milk right now.
I know those 3am feeds are knackering, but you're not alone!! I just try to tell myself that already he's grown up so much, that this time where he is a cute little baby will not last long. One day we will look back longingly at these days, so don't rush it.
I hope this helps :)