In my opinion, yes, you are starting way too soon.
Flash cards are well-used for 'review' for grade schoolers. You are wanting to introduce new concepts. To that end, here are some 'tried and true' ideas I've found useful over the many years I've worked with little people:
In my toddler group, when I worked on lesson plans, all of the information I introduced was through playtimes. Consider that play is the work of the child and how a child best learns about their world. You can do a lot of great, fun stuff through art, sensory activities and block-building, just to start.
For example: instead of a flash card with a picture of a triangle, cut out several triangles from paper and talk about them. You can make them different colors (and verbally identify each color : "red triangle" "blue triangle"); you can use clear contact paper and let him stick all the triangles onto it, then stick it onto a bigger piece of paper and hang it up. You can also do two sizes of triangles, big and small. Introducing comparative language.
You can use playdough with shape cutters. Playdough is fantastic in helping children with their hand (palmar/pincer grip) and will help with holding a pencil, manipulating scissors and other tools later on.
With letters, I suggest parents find a 'tray' alphabet puzzle (where each letter sits by itself in the frame) and use this for play. It's an investment which will last for years and worth the money. Having the individual letters to handle and play with is fun. My preschoolers loved playing 'cookie bakery' and using a spatula to move the 'letter cookies' onto small plates, trays or paper bags. Name the letters as you go. (Be aware that many children don't 'know' all of their letters by sight until 4 or 5.) Later, you can use those letters to spell out some words. My son is five and now uses it for reference when he wants to write something out but forgets what the letter's shape looks like.
Numbers: counting objects is great fun for kids. Line up a few blocks and point to them as you count them. You will do this seemingly hundreds of times before your son chimes in. This is the first step in counting, teaching 1:1 correspondence. There are also sets of Teddy Bear Counters which some kids really like. It's not just about introducing the number symbols, it's about teaching what each number 'means' (amountwise).
Another useful toy for when your son is a bit older are Pattern Blocks.
One of my toddler group's favorite activities was a gross motor "run to the shape"...I'd use painters tape on the carpet and make large shapes the kids could stand in. You can do this with letters, too. Use the tape to make one big letter for the day; you can run a car on the lines of the letter. This is about providing a 'whole body' experience of the letter/shapes.
Two books to inspire you: MaryAnn Kohl's "Preschool Art" and "Math Art". I think you'll like Math Art... it uses supplies you likely have at home already and will be far more engaging for *you* than flash cards. Also ask your librarian for activity books, ideas. Tana Hoban and Bruce McMillan also do great books for introducing language concepts, as well as Ann Morris, who does a lovely job of taking a simple concept and showing lots of varieties of that object from around the world.
Remember, too, that toddlers are learning a lot, it's just that it often isn't the stuff we're sweating about... but it's still important information they're taking in. So remember that the kind of instruction you are thinking of should come later rather than sooner. For attention span, we always calculated "one minute per year" and tried to keep activities open-ended, short and light. Lots of good storytime with you, hit the library for storytimes, try a music and movement class or a messy art class through your parks and rec.... keeping it fun and playful at this age is most important, and don't forget to watch him and discover what *he* finds interesting. Building blocks, running cars down ramps, water in a bin or sink with toys that sink and float... all of this is learning too.:)