Hold off on any fruits/treats for now. It's best to wait until his tummy has adjusted to the new greens first. Once he's on a daily diet that includes a variety of greens, then a bit of fruit can be introduced.
The lettuce you offer should be a dark green lettuce (romaine, red-leaf, green-leaf), not iceberg. If he's doing well with the small piece of lettuce, then offer a bit more of that same lettuce each day. Keep an eye on his poos. As long as there is no change (like mushy poo) then continue with that green each day.
After he's been on that green for about a week and a half, then you can try a small bit of a different green. Here's a guide on introducing greens (from my website) that includes a list of which greens are safe and which should be limited.
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