My FIL, FIL's brother BIL and DH all have Marfans. They are all tall (my DH being the shortest at 6'6). DH has the high palate, flat feet, arm span longer than height, big hands, hyper extendable joints etc. BIL has the long face, pigeon chest, muscle problems as well, I think his is more severe as he has to get an ECG once a year. It comes in varying degrees of severity and those with severe Marfans have a certain 'look' about them itms? But there isn't a specific genetic test for it as such. The doctor will just look for certain traits, check the eyes (retinas detatching) and heart (looking for the 'floppy' aorta). DH and all of his family are fine though, it doesn't affect their everyday lives, just something they need to keep an eye on.

DS had some issues with bow legs recently and we took him to the physio where he was diagnosed with hypotonia (low muscle tone, which is also a Marfans trait), the paed checked him out for it and we had to get his heart looked at too. As far as they can tell he doesn't have it. But the paed did say that some children are born with blaringly obvious traits whereas other will develop them over time.

If you are worried it wouldn't hurt to get your son check out...