I know how you feel about not having these experiences anymore. My husband doesn't want any more children so he got "fixed" about a year ago. I know I could have more if we really want to but I know our family is "perfect" now. We have one boy 8y/o and two girls 2 and 3 y/o. When I stopped nursing my youngest at 14 months I cried night and day. Every time I get my period I cry hopefully I will get used to it.
As for pumping if you have the time do it for mixing with formula and cereals then do it. As for the donating it I know some hospitals do this but I think you have to have a really healthy diet call and ask it can't hurt. I couldn't really pump it hurt and I was really uncomfortable doing it.
As for your son losing weight I don't believe there should be a chart to put our children on. My youngest is off the chart. I believe if your child is happy then it's okay if they are little or big for there age. My son is a tooth pick but the dr said he was a little heavy for his height and my middle children is too tall for her height. I think I'm just tired of all these charts.
Have fun with your son!
L.