I was then told that my son could be refused ILR if he didnt spend the entire 24 month period after the marriage with me...
Nobody here told you that.
At least not in this part of the forum. Mobility needs to be taken into account in this modern world.

Now about the VV. I have successfully represented clients who have received 2 year visitor visas. UKVisas must be willing to issue them if there's a checkbox on the form, right? Duh. In fact, the DSP's say that it eases the financial burden of applying if the application is for as long a period as justifyable.
But I've never seen one issued where the applicant didn't have 3 things going for them [1] a settled life in their home country; [2] prior travel history in and out of the UK; and [3] a very and very damned good reason for wanting one. So who fits that bill? Well, lots of people. Otherwise I wouldn't have anything to do.
Unfortunately bf's and gf's don't usually fit those criteria because being desperately in love with your bf doesn't qualify as a good reason for a 2 year VV. So they can get a standard 6 months VV. And if people in the North American consulates aren't doing their job, then tell Victoria or me about it. I'm taking it up with them at the next meeting anyway.
The other thing to remember about the 2 year VV is that each post has its own rule on them. And those particular rules are complicit with national security so they don't publish them. BUT just because a post has been given a procedure for the 2 year VV does NOT mean it will be refused - more often they will just issue a 6 month. So there's nothing to lose by applying as long as you meet the criteria.
Lastly. It's not clear if the OP understands this or not, but just to be clear: time on a VV is not bookable.
Hope that helps!