I agree, its probably age related and you just have to keep reinforcing sharing as he gets older but at this age I think distraction is the best thing, if he wants something someone else has distract him with a different toy. Im sure like all children there will be some jealousy and a big learning curve having to share you both with another little person but he will adjust. Don't be shocked if there are some tantrums and tears, praise the good behaviour, ignore the attention seeking and make sure he still gets one on one time.

And Nai is right second children are much better at sharing as they have never known anything different. My DD who is 3 tomorrow is much better at sharing than her 5 year old sister

Good luck with the impending birth of your DS and the changes in your families dynamics