You should also be aware of angiotensin receptor blocker (ARB) therapy. Click on my screenname and read previous posts on the topic, or Google the terms "losartan" and "Marfan." There is evidence that these drugs can halt or reverse aortic dilatation in MFS patients, perhaps precluding the need for surgery. This treatment is typically begun at lower aortic dimensions, however, so that's something you should consider discussing with a cardiologist who is knowledgeable about this therapy (not many are).
The threshold for recommending aortic root replacement is 5.0cm (or 50mm) or > 0.5 cm of dilatation per year. We look at all of the measurements of the aorta (that you reported above) when we use the threshold of 5.0cm. So, if he had dilatation of any of those areas of > 5.0 cm, we would recommend replacement. Also, we would look at how much aortic valve regurgitation ("leakiness") he has to see if he would need aortic valve replacement at the same time as aortic root replacement.
Hope that helps! Good Luck.