Sounds as if what you are doing now is the PERFECT answer, 'sharing' is a learned experience so it is much harder for children in a one child household, there is 4.5 yrs between my oldest & the next (now 42 & 31) and 3.5 between the oldest granson & his brother (17 & 13 now) that we are raising, the transition was difficult in the 'sharing' aspect, you just have to be, 'on it' all of the time that they are with other children & perhaps sitting down with him @ home, on the floor & 'playing' with some of his toys might help. I alwayz made the kidz share all of their things except for 'special' items that they NEVER had to share with anyone! That seemed to satisfy their 'mine' as it is with we adults, we have special things we choose to not share with others as well.
Good Luck, mama! You are doing just fine! :)