I'm 200 and have taken my two little guys with me for short rides (3 & 5 year olds), so I've put probably a total of 280 lbs on it.  Except for the stability issues with the kids moving around a bit, it floated fine.  You should be fine, but might be pushing it with your son on it also.

I think the question would really be whether you can balance all that on the board at the same time?   I am 6 foot 7 and 200lbs but never figured I would be able to take passengers on it?    Mine isn't done yet so time will tell I guess.

I paddle 6 days a week. Have over 800mile on my kaholo. I use it for testing paddles that I build. So at least 2 times a week I load my kaholo with rocks for testing paddles. The combined weight of me and rocks is 390 pound. The max weight I have used is 410 pound ,at that weight it is over loaded with standing area on deck awash and not so stable. With 390 pounds it is boarder line but a good weight for testing paddles. At 360 pound it is the most stable but it goes through heavey chop rather than over it. To any weight up to 360 pounds works good, Its still easy to paddle and stable and has good glide. There is not much difference between paddleing at 250 pounds and 350 pounds. If you are 275 pounds and your son is 60 pound the location for your son  siting will be just forward of the center frame of your board and you will be about 1 foot aft of the center frame. Your son can sit on the board first and you can start knee paddling and then stand up. It might take you a little practice learning how to stand up with him infront of you but once you are up its easy. My wife and my weigh combined is 360 pounds and it is no problem paddleing with her except she talks to much !!!

Ogre,

I like your building page!

410 pounds.  That's a minimum of about 200+ liters.  My wife gave me a thumbs up to buy a board.  I was looking at boards that were around 240ish.  But, what could be better than building my own.  So, I think I will go with the Kalolo.  Your response what just what I needed. I'm thinking about adding a mast track so will be seeking your advice in the future.

I cured my wife of wanting to go on the water when I took her for a Hobie Cat ride 20 years ago.  So, most rides will be silent. 

OneLove and Klaver, thanks for the advice.

Thanks Larry. 

Balance???  Two years ago I gave up trying to learn how to windsurf on a 1980ish Mistral after a couple of weeks of frustration.  The nail in the coffin was when a guy who never windsurfed before brought out his new gear.  His board was almost as wide as it was long.  HA!  Was I out of date.  I opted for an Laser II to get back onto the lake.  Now I'm looking for a way to play around on the beach without stranding my wife and kids on the beach for too long a time.  A SUP like the Kaholo or something like the Annapolis Wherry II might eliminate the rigging time and the need to find a suitable launch site for the Laser.  My ony fear with the Kaholo is that it might be a little narrow.  I don't plan to play in the waves with it but the boat chop on Lake Michigan and my balance might be an issue.  I'll probably build both but need to choose one ASAP.  My wife gave me a reluctant thumbs up to a project that doesn't concern fixing the house.