Impaction Is an Impaction in Our Future?

Hi- I have had my piggy for about 1 year now. Doodle is a 5 year old rescue; I don't know much about his previous owner, just that they let his teeth overgrow to the point he needed surgery. So long story short, he was abandoned, I don't know medical history until a year ago.

Since I've had him, I've noticed he's always had poos hanging out; I've never had a guinea pig before so I didn't think much of it. Lately, the poos have been getting less frequent and smaller in size. Usually they're clustered together like he passed 5 or 6 at once. He's now passing balls of cecal pellets the size of grapes. Appetite is great; free choice hay, Oxbow pellets, and fresh veggies twice daily, plus treats. He's very active; he does have small cage, but he is only in it 8 hours of the day when I am at school, otherwise, he is a free-range pig and has my bedroom to roam. Snuggles on the daily. I don't have another guinea pig, because I actually kind of stumbled into owning him; while I do enjoy owning him, I'm not in a position time/finance-wise to get a second pig.

He does get his sac cleaned once a week, and while I do find cecal matter in it occasionally, it's relatively clean. What else can I do to help avoid the impaction that I think is brewing? Is 5 too old to neuter and would it make a difference?

Other than the poo-problem, he's a very happy, chatty, hungry, spoiled pig.