Ultimately, it’s your decision, but I feel I have to address the other comments. You’re 3 yo will not be allergic to your milk (it’s not foreign to him and it is extremely rare that a child would be allergic to their mother’s milk – it’s all part and parcel of where they grew for 9 months!). It will not rot his teeth (my 11 month old, who is still nursed, has 8 teeth and I’m diligent, as with my 3 yo, to brush his teeth every day). You would produce enough milk for both children as it’s a supply and demand thing (the African American lady who had sextuplets nursed all 6 babies until 6 months old – she said she pumped 50-60 bottles a day!). Also, if you do start giving your older child breast milk, I can’t imagine you nursing him in public, and you never did say that you would put your older son to the breast – just re-introduce it. Also, you’re correct in thinking that your milk changes with the development of the baby, but I think it’s nominal and would really be the colostrum at the beginning.
All that being said, if you want to pump a cup or two for your older son, what’s the harm? However, you need to make sure you’re not unbalancing your milk for your baby – the foremilk is for thirst, then it’s the hind milk for calories etc. So if you wanted to give your older son milk, I’d pump after the baby had his meal.
I find it so funny that people “ewwww” over giving babies breast milk, from their mother, like nature intended, but find nothing wrong with drinking cow’s milk, from a mommy cow. I’m not a nursing nazi – my daughter weaned herself at 11 months, and my son is currently weaning himself – but feel that if you can nurse and pump, then give that benefit to your baby.
With that being said, I don’t think it’s a good idea to actually nurse your 3 yo (like I said before, a cup here and there wouldn’t be too bad). Although he’s eating food and perhaps taking a muti-vit, breast milk still contains hundreds of components that is still very beneficial for him. Very sick adults use breast milk storage banks to get the additional nutrients to build their immunity.
Now, from personal experience – when my son was born and my 2.5 yo daughter saw me nursing her little brother, she also wanted the “boo-boos” and tried to nurse. For a split second, I thought “well, what’s the harm” but I didn’t let it go any further than the thought. Kids are “no holds barred” and who knows what she would have said or done in public had I let her go back to nursing. I let her know that “boo-boo” milk was for the little baby and that she was a big girl. I never actually thought about pumping for her; I guess it never crossed my mind since she eats a well balanced diet.
But all-in-all, it’s your choice. Would I let my 3.5 yo nurse? No. Would I pump occasionally so he could have a cup here and there? Why not…but I don’t think I’d tell him where the milk came from. Also, breast milk lacks vitamin D, which is very important, so if you are going to do the breast milk route, make sure he has a vit D suppliment.