My DS aged 7 found his older brother's Captain Underpants book and loved them.
Anything similar with lots of illustrations and a little bit of reading. He usually doesn't sit down a read books and is put off with books with lots of writing.
We have Horrid Henry and he has read them (but didn't love them) and we have Diary of Wimpy Kid but I'm holding off on giving them till he's older.
Suggestions for heavily illustrated books in the style of CU and suitable for 7 year old.