Is there a reason you need it back on the left side? At 10 months, I'd just go with his preference and just nurse off the one side and not worry much about it. The one breast can absolutely produce enough milk for him.
I agree w/ Dana ... unless you have a reason for wanting the milk back on the other side, you and he will be fine nursing only on the preferred side.
That being said, if you do want to relactate the other side, you might consider fenugreek and blessed thistle, oatmeal, etc. You may want to pump more than just 3-4X ... but even at 4 X / day, it will still take at least a week and maybe more before you really begin to see a difference. Make sure you pump for at least 15 minutes. And if baby is willing and is just wanting to comfort nurse, you can try to put him to the breast on that side, which will help as well.
To reassure you, my DS2 nursed from my left side only from the age of around 16 months until he weaned at 30 months.
Sarah, mom to Alexis, Samantha and Kayleigh