Most babies I've seen don't really like being on their stomachs.  They want to be up and moving about like adults and big kids.  Has your son achieved some of the other developmental milestones such as being able to roll himself over?  You say he's standing, is he standing on his own or pulling himself up to standing position yet?  I've heard from more than one parent about their child that never really had a crawling stage and just went straight to walking.

The prone position (lying on your stomach) helps build  upper body strength.  Try rolling up a towel and placing it under his armpits to help him prop himself up.  Place a toy of interest near him.  Get down on the floor with him and encourage play in this position.  
 
It would be a good idea to check with your pediatrician who can determine his developmental level and make recommendations.

I think lots of babies get upset during "tummy time". It is really good for them and helps them build strength so it's important to keep encouraging them and trying to find ways to make it fun. But he's not alone in getting upset by it.
 
Many babies are moving around on their tummies and making efforts to get up to crawl by 8 months. But many aren't. I'd definitely share your concerns with your pediatrician to make sure everything's ok with your little one. But also try to remember that they all develop a little differently and it's ok if he doesn't do everything exactly when "the books" say he should.