Okay -- caveat emptor, I'm not an expert on Singapore. I've only seen bits and pieces of it.
But from what I've seen, it's still just the same math curriculum as almost everything else. What I've come to realize, is that 90% of all math curricula out there are basically the same thing, just with different "packaging". They might have different sequence, or a different reinforcement method, different pictures, different explanations, etc... but it's still the same basic method of teaching math. The pages of Singapore I've seen were virtually indistinguishable from the pages of the $3 math workbooks I used to get for DS in the regular bookstores, the ones that are just intended for "school enrichment" etc.
RightStart is a totally different method. And personally, I think it's stronger. My SIL is using it (level B) with her 7yo DD, they just started and they're enjoying it. I've just switched to it for DS who's now 10 -- he's been through the aforementioned 'school' workbooks, a year of Saxon, a year of Teaching Textbooks... he's technically finished grade 6 math, but I felt he still lacked true understanding in many areas. So we're doing level E in RightStart (basically grade 4) then doing their middle-school Geometric Approach. We just started, and it's easy review right now, but he's really enjoying it. He's very much a physical, hands-on kind of dude... I only wish we'd done something like this much earlier!
So you might guess what I'll say about your musing whether the physical instead of mental will help or confuse.
How can providing additional ways of thinking of math be harmful? If he understands mentally that 2+3=5, but is confused by doing it with real objects, then does he TRULY understand it? Isn't math derived from real life in the first place?
Using the abacus gives physical reinforcement, as well as visual, and the ultimate goal is to be able to do everything mentally by having an abacus in your mind (or just understanding the patterns more clearly if you're not as visually oriented).
The strategies of RS I think, are just so sound for creating real understanding. Like the author says, math should be only 5% memorization, and 95% understanding.
Now, as to your other concern about merging into school later... now it is true that RS is different enough that he'd probably have to do some 'conversion'. But if it gives a stronger foundation for true comprehension of how math works, then that foundation will stand no matter what methods he switches to down the line. I would think that at the worst, you'd just have to get a little workbook of the previous grade to quickly go through it together and see if there's anything to review or do differently or whatever.
But really I don't think it would be a problem. Everything for the first few grades is really SO basic, and there's so much review and re-covering in every grade, I don't think the transition would be impossible.
Heck, you might get lucky and find a private school that uses RightStart anyway
Or you could recommend to them that they switch lol...
Oh and I don't think the manipulatives will get tiresome either. They're FUN! Today we played "Go to the Dump", a card game that's basically like Go Fish except you're pairing cards that add to 10 instead of that match. Pretty easy stuff for a 10yo who's finished grade 6 math, right? Well, he asked to play it again THREE TIMES. And the math balance is just way too much fun... He's been experimenting with it on his own already, just trying to get the sides to balance by moving things around. And DD who's not even 2 plays with it.
She also plays with the square tiles - I just gave her a handful while DS and I were "working" and she kept herself occupied with her own explorations. While we played the card game, she took the unused cards and carried them around and "counted" randomly at us. I think it's great how she's entranced by the whole thing! And yes, I am planning to use RS with her officially in a couple years.
So that's my ringing endorsement for RS!