Cucumbers

Can't be very healthy for him to only eat one thing, especially something with such low nutritional value. I would try him on other blanched veg like courgette, carrot, broccoli stems, spinach, most beans... Just leave it in over night and see what disappears. I find green beans go down particularly well.

How long do you leave the wafers in? It can take plecs ages to track down their food and they sometimes take a while to recognize a new thing as food.

drop in some meats every once in a while. get him/her used to other things. this will help

lobster/crab bites are tasty snacks for most plecos i've found
and yes try other veggies.. DON'T complain though. the veggies are VERY VERY good for him