Dimples just my opinion but I would make sure at least one is available in his room. I would choose the one I liked the best! Or I would let him see them and see if gravitated towards one or two.
Later these items can be refereed to when birth parents came up in conversation. Perhaps if your county has an adoption day picnic or bbq that you go to or if you meet other friends with children who joined the family by adoption and find yourselves talking about his adoption.
I would say 'Blue Teddy or Tiger or Mrs Panda was a gift from your birth parents' or whatever.
If you put them all away out of sight it might be that in some way in the future he feels that part of his life is to be out of sight, and although I do feel it is private and not to be discussed with all and sundry, I also feel it is important for your son to know about it and for it not to be a shock when it first really registers with him.
Our son is a lot older, 4 now, 3 when he came to us. But I know as he becomes more and more ingrained into life with us it might be easy (for us) to feel his previous life was almost not real and he may even feel that at ties. The toys his birth parents gave him are on display and are very special to him, and to us. They are signs of the care his birth parents felt for him, or at least of their effort to make his birthday nice or to bring something to contact.
If a child had a very distressing past with birth parents I would maybe feel it was right to 'hide' some things or 'store them away'. So your own child's experiences may affect how appropriate my 'advice' is.
Of course my ds does remember birth parents (more from contact than actually living with them) and the gifts have some meaning for him but I would still feel that these gifts are a part of his history and need to be something age appropriate he can access. That is as long as there is no other 'issue' (unsafe/unhygienic/associated with negative memories etc) which could be a problem.