Add-ons to his Brio, Duplo, Toot Toot, Happyland collection or more stuff for sand/water play.
Art and craft stuff. Crayons, pens, sticker books. Play-doh. Nice apron/overall to protect his clothing. Easel.
"Everyday" role play - things to build off the kitchen set - some cuttable fruit, a tea set, more utensils - or expand it - pretend cleaning stuff, toy washing machine, doll and bath set, pram to push around. Shop set, toy till or shopping basket, etc.
Toy parking garage for cars or other small world stuff - maybe some dinosaur or animal figurines, pirate ship, aeroplane playset, farm,
More complicated jigsaws, large piece thick cardboard ones should be good now or soon. Number, colour or letter matching. Bead maze thing like you get in banks. More complicated shape sorter type thing. Giant beads with thick string to practice threading. Peg board. Very simple board games (e.g. Haba first board games, chunky snakes and ladders)
More exotic role play - dressing up stuff, doctor's or vet's kit, pretend tools in tool box.
Books, DVDs, story CDs
Bedding or posters to decorate his room
Annual pass to local attraction
Useful things themed for a character (book or TV) that he likes - plate/bowl/cup set, clothing, underwear, soap or shower gel, towel, flannel, food.