Ask your Pediatrician and a Lactation Consultant.
Also, ask your Pediatrician if it is "okay" that he is starting whole milk already.
Then, ask how many ounces he should be drinking, per day, of Breastmilk. Then ask about the whole milk and how many ounces of that, OR, if he should go onto Formula... in light of your decreased breastmilk output.
At this age, my son for example was drinking more than 6 ounces per sitting. That was for breastmilk. Then at about 1 year old he self-weaned and went to Formula.
So, your baby now gets 3 feedings a day, but he sleeps through the night. That may mean that he is fine with the current quantity he is getting. BUT, since your breastmilk output is decreasing... AND since a growth-spurt is coming up (typically at 12 months), you NEED to make sure, he will be getting adequate amounts, still. Because, at growth-spurts... their intake needs INCREASES and they feed more frequently.
Next, how does your son act at the breast? Frustrated? Pulling off? Not getting enough? Or just staying there as a pacifier? This may indicate to you, what HIS needs are....
Next, are you hydrated enough and drinking enough water, to keep milk production consistent? Do you eat well? Are you stressed? Or, it may simply mean that your body is decreasing milk production because your son is decreasing his time at the breast (direct breast nursing is the best way to increase milk production).
Keep trying to pump as well and more frequently. It cant' hurt and may help with your production.
Just some ideas and things you might want to consider.
All the best,
Susan