My daughter is currently 13 months old, I work full time and still BF. I'm very proud of us for having lasted this long but it wasn't easy. I returned to work at 8 weeks and she was exclusively BF until 4 months when I started to supplement because I couldn't keep up with her demand. Like other people have stated, make sure you have a great pump. You can get a brand new Medela on Ebay for at least $100 less than retail.
Also know that a baby can take much more milk out of your breast than a pump can, you have an average of 10-15 let downs when your son nurses and only 2-5 with a pump. The advice of pumping one side while you nurse the other to take advantage of the let downs and/or pumping after you've nursed to increase your supply is good. Drink lots of water, oatmeal is known to increase supply also. Fenugreek is an herbal supplement but you have to take 8-12 capsules a day (which I did but I don't know if it worked)and your urine smells like maple syrup. Reglan is supposed to help as well but it can cause severe depression and some OB's are reluctant to prescribe it. Some people look at pics of their baby or bring an article of their clothing to smell(sounds weird but it helps to relax the brain and gets you to think of your son.) I also bought a car adapter to pump while I drove to and from work.
Don't compare yourself to anyone else. I had 3 friends who easily pumped 8-10 oz in 5 min. It took me 15-20 min to pump 4-5 oz. The average person gets 2-4 oz per session, about 2 when you're first starting. My friends were over-producers and I was just "average" so it was very frustrating to compare myself to them, they had freezers stocked with milk while I had about a week supply. Contact a good lactation consultant for more info. Your milk will start and stop as you have let downs so make sure you're allowing your body to pump it all out, at least 15 minutes, you'll start to recognize how long it takes you after you've been pumping awhile. I also used the compression technique. During or after a let down I would use my hands to "squeeze" my boobs on all sides to hit all the ducts, I was amazed at how much extra would come out.
I also had a rule that I NEVER gave her a bottle, if she was with me, we nursed. The advice of pumping instead of BF to get your body is used to it is not very good, you need your son to take as much milk out as often as he can so that you're body still knows he's there. Nurse right before work, nurse at lunch if you can, nurse first thing when you get home and nurse all weekend. Use the weekends to get some extra pumping sessions in to increase your freezer stock. Your pumping will probably be best on Monday when you come from nursing all weekend and will probably be lowest on Friday after pumping all week. Nurse every chance you get to keep your supply up. Those that end up pumping only instead of nursing usually run out of milk.
Practice pumping and store as much milk as you can before you go back to work. Starting work again is a stressful time and that can also affect your supply. I ended up supplementing some formula between months 4-6 and it will be just fine if you have to use a little formula. But it will be so worth it if you can continue BF as well. You just have to decide how worth it, it is for you and stick with it. I stopped pumping when she was 12 months and she could take whole milk but she still nurses 2-3 times a day and we both still enjoy it. They are little for such a short time. Also, it's good to store as much in your freezer that you can in case you ever get sick and have to take meds, then you just pump and dump and give him the frozen. Contact me if you have any other ??'s. Good luck to you, it will be worth it!