Hi S.,
First and foremost, I would like to commend you on exclusively breastfeeding your son for 10.5 months, while pumping. I'm a SAHM and have never had to pump for long periods of time but I do know that pumping is a job in itself.
Secondly, as a mom that has exclusively breastfed all my babies up until their first year, gradually introduced solids and then allowed them to self wean, I consider myself a breastfeeding advocate and would like to share my opinion here.
To answer your question, how much formula do you need to give your son? I say, NONE! Our bodies are amazing and as you know your breastmilk has provided all the nutrients that your son needs up until now. There is no reason to think it won't continue to do that. Breastfeeding is supply and demand, meaning that your body will produce what you son is eating. You have been successful at pumping 2 oz when you pump daily, which, imo, is alot from pumping.
If you are nursing your son morning, after work and at night, your body is making the milk he needs at that time...it is highly possible that the 2oz you're getting while pumping is the most the pump can get so your body continues to make 2 oz during that time period, no more or less. By allowing your son to suckle longer at your breast when you nurse him, your body will meet his demand and make more milk.
Oatmeal, fenugreek and mother's milk tea are some products you can use to aid milk production. I am confident you can continue this nursing relationship up until his first year, if you're willing to continue pumping. Feel free to email me if you would like more information.
Best of luck with your decision,
Peace,
J.