Some plants Piggley loves are : anacharis , hornwart , frogbit , water lettuce , hyancinth all I let float. I get all my plants free at the city park but wash well and quarantine them first. You should do that even with store bought plants ! Trying to root them Al will just dig them up , turtle's are just to curious. Some are floating plant but I float all.
Now I ask my grocery store the manager of produce if I can make a grab bag at an agreed price for my turtle , most will say OK and "my store" gives it to me free ( a neighborhood store). It will be a one or two time thing , until you find what Al likes. This is to get one leaf of everything on the list below to see what will entice him the most. The only real trick is patience on your part. It's not to late at 2 years old to get veggies in and is a must. Your turtle is not a baby anymore it's a juvenile and needs around 50% veggies and by adult will be on 70% or more for it's health. All protein will lead to all kind of health problems in older turtles.
Read this :
http://www.redearslider.com/plants.htmlTurtles are like children that were allowed treats for dinner first. For Al before you feed any other food always start with the greens when he's the hungriest ! Stop what we call treat's ! They should only be offered one a month the most. Give him an hour to see if he eats any , then maybe two or three stick food only a day. Let say a leaf of red lettuce float in tank while at work if he changes his mine. The most for the protein side should be the head rule. I myself use the 2/3's rule a day not at each feeding , if you do it twice at 2" stop , only once a day , better in AM.
It's patience on your part and will not happen over night may take some time . But It Will Happen !