Unless it’s something extreme, a picture and/or brief description is not enough information for anyone to make determinations on whether a rabbit is in pain. You should consider behavioral signs, such as flinching, inappetite, lack of energy, reluctance to not be hunched, teeth grinding…. See here for one list of general symptoms that may let an owner know there might be something going on: https://wabbitwiki.com/wiki/Veterinary_emergencies

Do the treat test perhaps? If he isn’t interested in the treat or just sniffs it briefly without taking it, it’s a pretty reliable indication of discomfort/pain. (It doesn’t necessarily mean a lot of pain or pain due to something really bad, so the first thing to try is simethicone baby gas drops. But it means something isn’t right.)

If he was holding that position for a long time, then maybe. Squinting can be a sign. But if it was a just a brief moment, then prob not. Sometimes my buns will look like that when they are sleepy but about to groom, or about to eat a cecal.