At 5cm, his nutritional needs are pretty minimal. I would bet that he is grazing around the tank at night, keeping himself going. Give him time. In the end he will recognise algae wafers, and other veg that you might give him, as food items. You will also get to find out what he does,and does not like. As you are finding out, plecos' are not "instant" pets. They take quite a bit of settling in.
BTW zucchini is courgette in the UK. Another good veg to try is butternut squash ... cut a slice then give it 30 secs in a microwave. You will need something metal to weigh down any of the veg. I just pierce things with a teaspoon handle to keep them on the bottom.