Do you mean the poop is unformed, but very soft and sticky, sticking together in large clumps?
I think you can be certain he does have diarrhea, albeit a "midler" form of it than the potentially life-threatening watery puddles; the cause of this digestive upset is most likely linked to his diet. Do you leave the veggies in for a long time, or do you feed them and take them out after an hour or so if they are uneaten?
Hay is important, as you already know. He needs the fibre from this to help get his digestive system running efficiently again, to help stop the diarrhea. As Wiebke said, no veggies at all for at least 24 hours until his poop starts returning to normal - and until he gets some hay. Also make sure he is drinking some water.