I'll throw my tu'ppence worth in with a few alternatives to consider.
1st of all though, how competent a cyclist is your son Danny? Is he happy riding in traffic on main roads and what kind of bike will he be using?
As Yahuda has said, the direct route is the A635 and if commuting at 9 & 5 he will be going the opposite direction to the worst of the traffic. This gives about 6.5 miles each way but the roads are pretty awful dual lane affairs that cyclists are most vulnerable on as there isn't often the space for cars to pass without moving out a lane so they often don't bother!!!
If it is a direct road journey you are looking for then I would recommend
this alternative along the A662 through Droylesden and then the A6140 across the Ashton Moss which has a cyclepath for most of it's length if needed. The A662 is shared with the trams so you have to be careful of the tracks but as it is only single lane in either direction it is much more spacious and forgiving for a cyclist to have some space. Distance is a fraction more at about 6.8 miles
I would also consider the canal. It is direct and off-road and quite quick as well (well, it is for me

) but the down side is that once you get out past the MCFC and Velodrome it gets a bit mucky and is usually quite muddy around the Audenshaw/Guide Bridge areas.
My favoured option for someone not used to road riding would be
THIS as it is largely off-road but still should be quite clean in most circumstances as much of it has a hard well drained surface that will be solid throughout the year. It is a bit further at 8.3 miles but a more enjoyable ride altogether IMO.
Best idea I can suggest is for him to try them all and then make a choice or use a mixture of all the suggested routes perhaps the clean one on the way to work and the mucky canal blast on the way home afterwards. Just another thought, some of the off-road routes are most likely unlit so might be problematic during the winter months?
If I can be any further help just ask, I might even be able to do a practice run with him if it would help