Hi S.,
You don't have to start cereal yet. If your boy is doing well, you may want to consider just breastfeeding for a while longer until he exhibits signs of wanting solid food - usually that happens around 6 months. Of course, every child is different and you're his mother - so whatever decision you make is going to be the right one for your family and child.
If you're concerned about your supply, head on over to Whole Foods or another natural grocery and find More Milk Plus by Motherlove. I struggled with supply for all 13 months that I breastfed my daughter, and it's the only milk supply booster that has blessed thistle - which I found way more effective than Fenugreek. It's a tincture, so you put it in a small amount of water and grimace it down. You may smell a little like maple syrup while you're taking it, but your supply will increase. You could become engorged if you take this when you don't need to - so go slowly while you figure out if you require it.
Also, a gentler way to boost your supply is to think about eating oatmeal every day. Oatmeal supports lactation. Other solutions: skin to skin contact with your child - take a bath with him. And, if you feel your supply waning, nurse him more. You can also buy Nursing Mother tea that supports lactation.
If you haven't had supply problems to this point, there is no reason to think that problems will start now. If you're a normal milk producer, your body will keep up with demand naturally. If you do have problems, just one or two of these things will likely keep you in enough milk to feed an army!
Also, if you're concerned about pumping, maybe you should try and pump now to get a frozen supply of milk as a backup so that you don't have to worry about it. Of course, fresh milk is best - but frozen breast milk is sure handy to have around if you need it.
Good luck, and happy breastfeeding!
T.