What do you mean by he 'got out of his pen'?
Did he get out into somewhere where he could have eaten a whole bunch of grass, or where he could have gotten into your feed room? You got any bagged feed where he might have gotten some? Say in another stall or room he might have gotten into?
Was he out for a long time? Maybe over night? What is around where he got out, that he could have gotten in to? You got any other horses or animals around? Are they sick? Did any other horses come and go off the property recently.
Is there any chance he could have eaten something different than he usually eats? Amount or type of food?
Was there anything different about the last couple of days? Could he have gotten more or less exercise the last couple days?
Has he made the usual amount of manure?
What I would suggest you do is call the vet and have him/her come over and find out what is wrong.
Since it is an older horse, and since it got out of its pen, since the horse did not eat its feed and did not eat its hay, and MIGHT have gotten into some grass or feed, there are a couple things this could easily be, but you need a vet out there.
A couple more questions - is he standing in a funny position? Does it look like he is trying to get weight off his front legs? Is he standing with his front legs stuck out in front of him or otherwise seem to be standing funny?
Take his temperature, pulse and heart rate. You got a stethoscope and know how to take pulse and heartbeat? Can you take his temperature?
Is he trying to lie down, look back at his belly? Groaning? Any sweat on him, even a fine sweat just on the end of the hairs? Especially check his face and forehead for sweat.
Does he seem in pain?
When you say grouchy, do you mean when you try to get him to move he don't want to move, or what exactly?
Ideal is you get a vet out right away, but if you don't have the money or can't people can try to help but no one can do as good a job as a vet who is there and can see the horse.
If he got out and overdosed himself on grass or bagged feed, I mean really ate a lot of feed, my worry would be laminitis. His pulse, heart rate would be up higher than normal and you might even feel his feet seem too warm to the touch (on the hoof).
If he is just having a belly ache (colic) that can be due to not moving around for a while, not drinking enough water, coarse hay, etc. Colic can be mild or severe. Most people would try to treat mild colic themselves. They would walk the horse for a couple hours and see if he was doing better, maybe give some banamine paste by mouth(it is given by weight so you would need a weight tape to estimate his weight).
But those are all guesses. He could have an infectious disease, it could be a lot of different things. Vet is best.