Nice looking pup!

Well it seems like you've got the basics down sit,stay,fetch.
If you can teach those you can teach heal. Hes at a great age still very willing and excited to learn.

Iam working with non hunting hyper house lab(2yrs) right now!
It only took her a couple days to catch on, very smart just never been worked with.
Anyways, this what we do
Take a slip lead, make sure its in the proper spot(under the jawbone placed right against the base of the skull ). It doesn't have to be choking them. Get the dog in position at what ever side you prefer have them sit to begin, start walking(give the HEAL command) keeping the dog snug to your side. As you walk hold your arm out keeping the lead straight up in the air, when he starts to stray give the heal command along with a quick pop up on the lead. Remember heal should be with the dogs head at your heal not leading infront or lagging behind. I walk our dogs up in down our driveway its nice and long, stopping every 50ft or so, command sit when you stop. It should only take a cpouple times and your dog should be able to read you and sit when you stop.
Repeat Repeat Repeat.
Take about 15 mins everyday
Before long your dog will be healing to you with out any commands
Also if you'd like add a whistle, use everytime you stop to get the sit on whistle trick down.

Hope this helps, its just the way we do it. Works for us.
Look around take advise from many people and find the method you like the best for your dog!
Enjoy that pup