Hi, I'm so sorry that your baby boy has had to struggle so early in life like he has, but I'm happy to hear that he has a brother who is a perfect match.
Yay! As for your questions, to keep an eye on my hemoglobin, I've always checked the lines in my hands. With his fingers straight and his palm up, push his all fingers back just slightly so that you can easily see the lines in the palm of his hand. If the lines turn red quickly when you push the fingers back, then his hemoglobin is ok. If it takes a while for the lines to turn and the color is pink or maybe no change in color at all, he's low. You can also watch the color of his nail beds. They will be pale when he is low and when any pressure is applied to his fingertips, they will turn light pink or even white instead of immediately turning red. For all these examples, compare to your own to get a good idea of what normal would look like beforehand.
As for the blood transfusions, they don't hurt at all. If he doesn't have a port, the IV stick to get the transfusion can stink sometimes, but the actual transfusion itself doesn't hurt a bit. The only thing that can be bothersome during the transfusion is that I would get really cold. If the IV was in my hand, I would ask for a warmed towel to wrap my hand in. If the IV was in my arm or after I got the port, it was in my port, I always asked for a blanket to be warmed and thrown on me. The blood in the bag is kept pretty cold and as it goes into your veins, you start to get really cold. Other than that, nothing much else to report on that front.
I'll be sending blessings your way for your sweet little boys.