Kids tend to be very proprietary that they sometime make a fuss whenever their toys or stuffs are used or touched by another child. In fact, they tend to be over proprietary, they would think anything they touch is theirs! On the other hand, especially for little children at pre-school, it is very easy to lose their things given that they do not know how to read that much and it is easier to confuse their books and other school things with that of their classmates.
A nifty solution to solve this recurring problem is to put personalized tags on each of the kiddos belongings and the trick to make it work is to put these tags on prominent areas your little ones can spot easily. You might add in a symbol or a photo he can clearly recognize, like his favorite cartoon character, for example, so that he might be able to tell which are his even if he cannot read that well just yet.
I bet the little ones will enjoy putting tags and names on their things, the next thing you know, he might just be requesting desk name plates for him and his classmates to use in class.