A few things that have worked for us--
-Sit down and do the puzzle with him, guide him with directives, questions, and encouragement.
-Let him watch you do the puzzle sometimes, it will help him learn and build interest.
-I second the suggestion to put together the puzzle with leaving some pieces out for the child to fit in, what a great idea!
-We have some jigsaw puzzles (cardboard- 4 pieces with a frame) with animals or crayons on each piece. I used a labelmaker to label the frame/backing with the color or animal name to serve as a hint.
-I used a marker to do the same with some other puzzles to reinforce reading skills and the # of sides on various shapes. I'll take some pictures and upload them soon.
-Try to introduce strategy, specifically pointing out edges and corners. My Dd still doesn't get this 100% but we work at it, it will be very useful as the level of pieces grows.
-When putting together a dog puzzle for example, say What is the doggy missing on his head? Ears? Okay let's find his brown ears! or...Oh look, paw and paw! eye and eye! tail and tail! it helps them figure out what is missing in the picture to determine what comes next. Of course you can also teach them to look at the finished image on the box for clues.
-You can also separate the edges from the middles completely, just leave the middles in the box. So that 24 piece puzzle becomes a 12 piece edge-only puzzle the child can assemble first. Once the edges are complete THEN take the middles out of the box.
-Teach your child to put the flat edges of the puzzle on a flat surface like the floor to determine if it's a middle or an edge. My daughter had (and still does somewhat) have a hard time discerning an edge from a middle piece.
-Let him solve the same puzzles again and again. His confidence and fine motor skills will grow. My DD did this and was able to figure out the puzzles because she knew the "picture" by heart, not because she was "solving" in the truest sense. It is still great developmentally to let him memorize the images on the puzzle and where they go, the way the pieces interlock will become more and more natural over time. Eventually those skills will grow and develop into the ability to solve new puzzles, I compare it to figuring out phonics. At first they might only recognize the words they have been taught, but eventually they can read new words they have never seen before. Same concept!
-I also agree with the suggestion of big floor puzzles. When we jumped from 12 pieces to 24 pieces, we opted for the big Melissa and Doug ones. The graphics were simple and the big pieces were easier for little hands to manipulate. But the over sized pieces are just a fun novelty, just like big text in a book.
-Expect the process to be very hands on in the beginning. Each time, offer the child more control and independence as appropriate, helping them out with suggestions when they get stuck.
-Try to avoid puzzles with lots of pieces that look the same. For example, a puzzle with pieces that all have black and orange tiger stripes will be a lot more difficult than a puzzle with many different images & colors to match up that is easier to differentiate.
Sorry if some of these suggestions are obvious, I just figured I'd include them all! Just keep working at it, and he will learn with practice! And above all keep it fun!
http://www.amazon.com/Melissa-Doug-Happy-Harvest-Puzzle/dp/B00146K0JA (24 piece floor puzzle, excellent quality, the pets one is great too)