Around that age I started reading Harry Potter, and my son is now an HP fanatic. The only problem is that we got through them quickly, and I'm not entirely thrilled with reading past Book 5 to a first grader. Something to consider.
James and the Giant Peach would be great. When he's a bit older, try the Lemony Snicket series too.
Oh, and his friends are into a series called The 39 Clues. You could read those to him now, and then he could read them himself within a couple of years.
The books you mentioned are picture books, so I'm assuming you are looking for more picture book series?
The Trucktown books are good picture books for boys.
Diary of a... (Worm, Spider) books are gender neutral picture books, and are pretty funny.
Skippyjon Jones is gender neutral and hilarious.
Arthur is another good neutral book series.
My son loves to learn about things, so any DK books or Magic School Bus books are great for him. As are any non-fiction books, really. He likes reading about sharks, Egypt, the ocean, the Olympics, volcanoes, caves, rocks, animals, anything really.
If you are looking for chapter books, Magic Treehouse series books are gender neutral.
Nate the Great is a good beginner chapter book series for boys as well.
We have a bunch of Magic Treehouse as audio book that DS has loved since he was 4. We are reading bigger books together and so far he has loved The Mouse and the Motorcycle, The BFG, and Mrs. Frisby and the Rats of Nimh.
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Lots to check out, Thanks!!! He is still very much into pictures in books so although the story can be on the long side, he expects pictures still. We have several 1st chapter books from Usborne (with pictures) and he's happy with those. We have, Sorcerers Apprentice and The Dinosaur Who Lost His Roar. We also have the Usborne book on Pompeii which he's really into and it's really long (for a bed time story). I was worried the subject would be too much for him but he's a logical thinker and just sees it as it is.
DS loves the Pugwash series - Pugwash is a pirate (a bumbling one, very non threatening). There are quite a few in the series, and they have great illustrations. Perfect for 4-6 age group.
Any Julia Donaldson book is also good. While they aren't a strict series, they follow a theme (clever rhymes, good plots, similar style of illustration). The Gruffalo is a favourite in our house.
The Zen Shorts series (Zen ties, Zen Ghosts)are also excellent. A good introduction to philosophy, and lots to discuss with absolutely beautiful illustrations.
This one isn't a series, but is so good on its own - The Giant Jam Sandwich. Clever story, great rhyme and detailed illustrations (my son loved the machines hovering over the field, like a tractor suspended by balloons).
I would avoid chapter books at 4 (and there are plenty of chapter book series for 6+ year olds). He'll need illustrations to keep his interest, and developmentally it's better as well. The books above are all on the longer side and more plot driven, so not picture books for a 2 year old.
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I was going to recommend The Edge Chronicles but I think your DS is too young (there are some darker themes). However, I recommend you add it to your list for when he's older because it's a fun series.