Mine are that far apart. My toddler was an easy one though.
If I were you I'd make a super safe area of the house and gate-lock it off so that you can all be in there when you are nursing the baby, or getting the baby down for a nap.
He will eventually understand consequences but with a toddler that active it is going to be a hard year when going out :-( You could use one of those kid leashes, sling the baby and carry the toddler, make him stay in a stroller, or sign him up for one of those preschool-daycare out fits that run just 2-3 mornings a week for the real little guys, or save your errands for when dh (or someone) can be at home with him.
I think he probably can understand consequences if you are very very consistent. He might be to impulsive to comply or he might not understand the rules-parameters. The rules have to be easy to understand and visible. For instance at the doctors office you might carry a small section of rope in your purse and then lay that on the ground in a 4' circle around you. He can play w-toys in that circle. I saw a lady at church do something similar and her toddler stopped running up and down the aisles.
Can you afford a mothers helper for a few months? A teen to come in for a couple hours in the afternoon to take him out to play or something?
-- Nikki
