It's a proven fact that having many toys around can have negative effects by many well known child psychologists. I do agree that only one toy at a time is a little much, perhaps having a small corner of a few toys would be more ideal for his age and store the rest in his room.
For example, I have blocks out, but my children don't touch them when their toy basket is out because they have so much to wade through and they bore of everything easily and get irritated... But, if I put the toys away, and pull out the blocks or a puzzle, they have so much fun concentrating and playing with them and using their imaginations. My kids are the same age as yours.
Also, at 2.5 your son is old enough to get his toys and put them away on his own, he may need to be shown several times and have help at first, but with kids his age, they love to stack, sort, dump things out and put them back, So, in itself, putting the toy away and grabbing another could very well be like a game.
The Guardian recently published an article examining the effects of children having too many toys. As you can probably guess, the results are not positive. The findings were that an overload of toys can possibly hinder child development and creativity. Also, having too many toys can affect the child’s attention span and leave them giving up on an item quickly.
You may understand a little better from this article:
"One of the greatest myths perpetuated by schools and the media is that young children do not have long attention spans. Nothing could be further from the truth. In the last few decades, we have inadvertently crippled the natural concentration of young children through television’s chopped up programming, too many toys that require no creative thinking, and segmented public education. Then we mistakenly call the existing deficit in their ability to concentrate inherent.
In order to observe the actual attention span of a young child, we only need to watch children when free to use their time as they wish, without television, and with no one to design the day for them or break it into rigidly structured segments. They are absorbed in whatever they do for hours and days, forgetting to eat and often refusing to go to sleep!
Have you ever read to your child the same story or page over and over again? Who got tired first - you or the child? If we keep responding to the child’s need for repetition, we will support his innate ability to concentrate on one task. Long attention span is the child’s nature, as long as she pursues her own passions without being interrupted by school bells or by our own agenda for her. Even dinner and bedtime are not holy and can give way to respecting the child’s innate guide.
The following are some of the common robbers of children’s innate ability to focus, and guidelines on how to use these same resources productively:"
(ideas in link)