Some pumping tricks I learned when pumping exclusively for my non nursing infants may help you.
1. Continue to pump for an additional 3-5 minutes after the flow stops. Many times you will tap into a reserve supply of milk (up to a couple of ounces) and when you this tells your body that the baby is not getting enough and it will increase supply. If you do this every time you pump it can really make a big difference.
2. Cluster pumping. A couple times a day (if you can manage) try pumping/feeding every hour, while at the same time continuing to pump those extra 3-5 minutes each time. This is especially helpful if you do it in the early morning when supply is highest.
3. Never go more than 4 hours without pumping or feeding. I used to rest from pumping only from midnight until 4 a.m.
These tricks helped me to nearly double my production. I don't know if it will help you as much as it helped me, but I hope it will increase your supply at least some.
It's really common (and normal) for baby to cluster feed in the evening. It's baby's way of increasing your supply. You might want to stop the evening bottle and just feed baby at the breast ALL EVENING LONG (it seems sooooooooooo long - I used to watch DVDs to pass the time) and in a couple of days mom's supply usually matches baby's need.
Here's a good explanation: http://www.kellymom.com/babyconcerns/fussy-evening.html
and http://kellymom.com/bf/normal/growth-spurt.html . ~6 weeks is a common time for a growth spurt.
I also had to watch movies and breastfeed all evening when by son was that age! He would just switch from boob to boob and nurse for hours around that time of day.
Don't assume, just because your breasts feel softer in the evenings, that there is no milk in there! Just because a pump isn't able to get much doesn't mean the baby can't. Why don't you try cluster feeding your son for a few evenings without the bottles and see if that helps? Right now, your boobs are making exactly the right amount of milk for a baby who has two bottles. You need to get them making exactly the right amount of milk for a baby that has no bottles, and the only way he knows how to do that is by sucking constantly on your "empty" breasts.