I am going through a similar thing, although my 3 month-old babe is still nursing quite a bit (in the lying down position - we are co-sleepers). But, like yours, ever since he started really noticing things, he wants to be looking around and checking everything out. It is hard to impossible to get him to nurse in the crade position that we've done for so long because he's constantly squirming and trying to look everywhere but the boob. I don't know about you, but nursing in public is never possible these days, which makes going out with him a bit tough because I know he'll eat if he's really hungry, but I don't like pushing him to that edge.
He also loves being on his tummy and trying to crawl or stand (to the point of total exhuastion), so he's basically always trying to do that - hard to be able to just sit and be mellow - I feel your pain! And it's such at the in-between stage because he's not mobile enough to actually "play" but he's not just a sit and nurse baby anymore either. I feel like I am constantly trying to find things for us to do that will make him happy and not completey wear me out since I am home with him all day. And, my guy is definitely teething too, so you put all of that together (plus just the normal growing, gas etc.) and I think it makes for, like you said, an overstimulated baby. I use that Hyland's teething gel which does seem to calm him. Just now he was having a hard time so we nursed a bit, tried to crawl a bit, I tried to lay him on my chest to sleep and finally after nothing really seemed to be making him happy, I put him on his stomach and rubbed his back and it knocked him out. I have used that technique the past two days and it has worked so well. When I massage his back, I go in a counter-clockwise motion covering the entire back and buns (I was a massage therapist, so I know it's important to keep things moving the right way for digestion) and I do it somewhat fast and vigorously at first just to distract him from his woes and then as his eyelids close, I slow it down a bit until I stop.
Also, with this whole teething thing, I am getting bitten on the nipple more than I'd like - how about you? Even though it hurts, I feel for the guy since teething can't be fun (good thing we don't remember it as babies).
Everyone tells me this will pass and I know it will. Once he can actually crawl on his own and is more mobile, he'll be less frustrated (I hope) and teething, of course, is just a phase. But, I know how you feel - friends have asked me lately, "So, are you coming out of the baby fog?" and I say, "He's been so good, I was never really in it... until now!"
Hang in there!