Congratulations on your new little one, and welcome to the forum!
You say that you believe you have a great deal of milk ... do you think that you could have an oversupply that your son is having a hard time keeping up with? Babies dealing with an oversupply will typically gain weight very rapidly as yours has, and will fuss and cry when your milk lets down if you have an overactive let down. Does milk spray when he comes off the breast? Maybe take a look at these helpful pages and see if anything rings a bell for you, then post back and we'll all go from there.
Am I making too much milk?
Oversupply and OALD
My baby fusses or cries during nursing - what's the problem?
Regarding oversupply and overactive let down LLL's page says the following:
"When a mother has more milk than her baby can handle, the following behaviors may be common:
Baby cries a lot, and is often very irritable and/or restless
Baby may sometimes gulp, choke, sputter, or cough during feedings at breast
Baby may seem to bite or clamp down on the nipple while feeding
Milk sprays when baby comes off, especially at the beginning of a feeding
Mother may have sore nipples
Baby may arch and hold himself very stiffly, sometimes screaming
Feedings often seem like battles, with baby nursing fitfully on and off
Feedings may be short, lasting only 5 or 10 minutes total
Baby may seem to have a "love-hate" relationship with the breast
Baby may burp or pass gas frequently between feedings, tending to spit up a lot
Baby may have green, watery or foamy, explosive stools
Mother's breasts feel very full most of the time
Mother may have frequent plugged ducts, which can sometimes lead to mastitis (breast infection)
If many of these experiences seem familiar to you, it may be because you have an overabundant supply of milk, which can cause a forceful milk ejection (sometimes referred to as overactive let-down), and/or foremilk-hindmilk imbalance. The infant behaviors described above are caused by these issues but may frequently be misdiagnosed as colic, lactose intolerance, milk protein allergy, reflux, or hypertonicity (stiff muscle tone)."