I have never heard of avoiding spinach until a certain age. Oxalic acid can interfere with iron absoprtion, and iron is very, very important for developing brains - so my guess is that's where the recommendation you read or heard came from. However, I can't imagine it being a problem unless you are giving him lots of spinach and not much in the way of iron. If you are bfing he would almost certainly be getting enough iron.

Whether you are bfing or not, meat is now recommended as one of the first foods to introduce (because it is high in both iron and zinc which is also important for developing brains). For young babies with few or no teeth, it can be easiest to give steak to suck on, safe bones to chew on, or to scrape the meat into a pulp and add to other foods.