Is the lump hard or soft? Is is poking out of his belly or on his skin/anywhere else?
Kale is safe for dragons.
He may have just eating too much at once and have his belly poking out because he is full, my baby gets this sometimes, is usually nothing to worry about. If its passed in about 2 days then I wouldnt worry about it.
Is he showing any other signs, like unwillingness to eat, or slow/lethargic behavior?
What substrate has he got in his viv?
Try giving him a warm
bath for 15 mins, (water up to his armpits) and rub his tummy, this should help with any digestion issues if present