I nursed my daughter until SHE self-weaned, at about 2.5 years old. However, she never expressed a frustration with any diminished milk supply.
Mind you, at this stage, you will not feel a "let-down" per say. At least I didn't, but I knew I had milk still, I asked my daughter and she said I did. And when I squeezed my nipples, milk would come out.
Also, your son is only "nursing" twice a day.. thus, the diminished milk supply.
You can consult a Lactation Consultant about "increasing" your milk supply...but you will still need to come to a decision about when you want to wean him, or let him wean himself. Hypothetically, say you DO increase your milk supply... what then? Will you go back to nursing him more? Or less? Or STILL want to wean him, or let him wean himself?
Then the question is, do you want to do this for yourself, or for him? Weaning is bittersweet... but for both my kids, I let them self-wean. And they did. And it was fine. For me, and my Hubby, self-weaning was something we both believed in.
You can also do a google search and research "extended breastfeeding." This is what it is called.
You mentioned that you feel depressed about it and sad. That happens sometimes... but our children will grow up just fine, when weaned. And, as a Mom, we have to let them. But your son still seems upset about your diminished supply... but kids are adaptable, and he MAY get used to it.
But the thing is, you will have to find some sort of peace about it... and even if you DO increase your supply... then what? And, how long do you want to nurse? Sometimes, "our" time-line and hopes about it is not pertinent... because the child has their own level of "conclusion" about it or not, and their own timing.
Since you only breastfeed twice... that is a type of "weaning." And it may be that he is just upset over your body's natural diminishing of milk. Which does happen, as you nurse less and less. The body adjusts to the frequency.
But don't worry, it won't 'harm' him or traumatize him. But I think that a child will feel the anxiety a Mom has about it... and may react to that.
*as a side note: do you know for SURE that he is unhappy at the breast because of "diminished" milk supply? Or is it because, possibly, he does not want to nurse anymore??? The reason I say this is because my son at about 12 months old, just protested/grumbled whenever I'd put him to breast.. ...and he just wanted a bottle. I still had tons of milk. But, he had, on his own, decided he no longer wanted to nurse. He weaned himself. I took HIS cues, and I finally realized what he was telling me. Then he was much happier.
Only you can decide, and your son. Don't feel pressured by you other family members telling you to quit or not. YOU are the Mommy.
All the best,
Susan