I understand both sides of what is being said here. I can see that lilo you are affectionate with your DC, and you have experience.
Tortoise, I guees this is a first for you, so you want to make sure you are doing the right thing by your son, and yourself!
Have you tried explaining to him that is not because you dont love him, or that you love him less than your other DC, it is simply a physical thing. That it makes you very very sad that you cannot pick him up anymore, that you would love to, if you were strong enough, but you just cant. That it also makes you so sad to know he's upset by it, and could he try as your biggest boy, to understand for you?
I don't know, it might be worth a try? Ham up the whole 'being mums biggest ,strongest boy' bit, maybe it'll help him feel important? maybe even ask him to join in when you are doing this to your other DC, could he actually hold them at all? you know, bringing his role as the eldest into it, as your number one support kind of thing? Just really emphasise how you wish you could...
HTH a teeny bit!