Messing around with your fin(s) will definitely give you a boost, but it sounds like you need to walk the deck a little more. When you start to slow down, take a step or two towards the nose and keep your feet close together. You're going to kill your speed by keeping a wide stance and if you just hang out in one spot. You should definitely get some new life out of your ride by adding a little footwork and tightening up your stance. You may already be doing this - who knows.
Your fin - what kind of set up? Do you have a Magic? Tri-fin? Unless your in tall water with clean faces, I don't really see a "need" for a tri-fin set up. If you have side bites, drop them. Get a 9" fin - RFC Wingnut is a great fin to get the ball rolling and remains one of my favorite longboard fins of all time. Start with your fin all the way towards your tail. Try it out and adjust by 1/4" incriments or so. I wouldn't advise positioning your fin over the "half-way" mark of your fin box, but that's just me.
Steve makes great boards and I'm sure it will be only a matter of time before your tearing it up out there with your son. Best of luck.
Waldens have a lot of rocker-- they are more performance longboards. For really small waves, easy wave catching you want a more old school longboard-- flat rocker. Walden will be better on bigger, faster waves, old school longboard on smaller slower waves.
-Start paddling earlier. Takes an extra stroke or 2 or 3.. to get a big tanker moving, but once moving should catch waves easy.
-If your on the fringe of catching a wave, kick your feet towards the back of your head. Sounds and looks odd, but always forks for me on a longboard.
-When your up and riding, move fwd to trim. walk shuffle whatever, just move around. If you just plant your feet and go, yourdoing it wrong on a longboard.
I don't ride longboards too much any more, but used to ride them a lot. I still have to get back in the groove when I do ride one, even having the experience. I bet you'll like it eventually, but longboards are definitely different. Even though your other board is "shaped like a longboard" it's too short to really feel/respond like one in my opinion, so there's a learning curve.
I recently custom ordered a Walden and took delivery in Dec. 10X25X4. Took me two, 4 hour sessions to figure it out. Cold March water and 5/4 suit, hood, gloves and boots didn't help. The board will turn instantly if you take a step back, then step front. At first I found it to turn too easily, as my other board was a 12 footer and had to throw my weight back to get the nose up and around to get it to turn. Up until a couple trips ago, I would bring and ride both boards. The last two, only the Walden. Now I love it! Had it out Sat, Sun and today. Yesterday rocked with a really nice swell and my new Walden took me on some nice rides. When dropping in, just lean back on your back foot and you're headed down the line.
Oh almost forgot one thing... once you catch the wave, you gotta get further back on the tail and use your weight / foot placement to make that board turn, it's probably one of the biggest issues people have with the longer boards.
This serious issue is entirely due to endemic poor design.