What is the best harness for a lab? And for an 18 week old should we get a large or get the x-large that he will need later because he will be about 100 pounds.
I would get the size that your dog needs now, ensure a good fit. If and when he grows out of it, buy another. You will get lots of harness suggestions. The Perfect Fit is a good one and is made up of 3 separate pieces to ensure good fit, and also when the dog grows you may only need to buy one of two new pieces. It has a front and back attachment points. I had this harness for my previous Lab, Scooby. He was a large dog and we went for the 40mm width (they do 25mm too). Coco is a much smaller dog and the 40mm harness didn't suit him, too chunky. We use a Haqihana harness for Coco which is a lovely fit, though it lacks the front attachment. It appears to be a very comfortable harness and Coco walks well in it.
Perfect Fits are excellent harnesses and I highly recommend those. If you are having trouble with pulling or lunging or control, I think the Freedom harness has the edge in control.