So he throws toys outside, or inside too? Have you shown him things that should be thrown? Do you find that he expects a certain reaction from you? What would happen if you reacted in a different manner? Has anyone gotten hurt. Anything broken?...
Just trying to get more info----
Any new stressors, changes in his life?

He throws toys inside and outside and he throws sand outside everytime we play in his sandbox. I have shown him that he can throw balls outside in the yard. I never thought about him expecting a certain reaction but yeah, he probably enjoys the attention that he's getting although I spend a lot of time with him. I don't really know how to change the way I react to him. Right now I sternly tell him not to throw toys, tell him why it's being taken away, etc. Nobody has gotten hurt but he has hit the cat with toys. He also hasn't broken anything- I just lose patience having to repeatedly tell him not to throw toys. DH is Navy and works long hours along with coming and going every few weeks for a few weeks at a time. I'm sure this has something to do with his behavior but I don't know what I can do. Thanks for your help.

Maybe you could try just letting him throw as much as possible in a pre-safed area. Outside, get the cat out of the way etc. Maybe he--enjoys it---gets his anger out---expresses his feelings towards you----etc. He certainly sounds like this 'dance' between the two of you has become a fixture in your life.

I would try something new or different to break the rut you both are in. You feeling helpless to stop it, angry etc. He needing this outlet, feeling your energy etc...

From www.askdrsears.com
"Choose your battles wisely
At this age, toddlers are incapable of following every rule about safety, social behavior, and family life. If you try to enforce every rule on an 18-month-old that you would on a three-year-old, then your whole day will be spent punishing your child. Your child won't learn to follow rules; he will only learn that life is full of fights and punishments. We suggest that you choose which rules that you feel are the most important, and decide which rules or boundaries are minor ones that are not yet worth fighting over. For example, perhaps it is important for a toddler to learn not to let go of your hand and run into the street, or not to bang on the sliding glass door with a hard object. These are pretty important rules. However, if your toddler likes to bang on the wall with his plastic toy hammer, or wants to let go of your hand to run around in your safe driveway, these are things that might be slightly annoying to you or border on being unsafe, but are probably not worth fighting over. By giving your child the freedom to explore, try new things, make some noise, and get a little messy, you are simply allowing him to be a toddler. Let the little things go. Concentrate on the big things. You will be able to spend more time playing with your toddler than disciplining him. "

throwing is developmental, just like teething, weaning, etc. he NEEDS to throw. he's learning and exploring. the little scientist is learning the sounds things make when they're thrown, he's developing his muscles and coordination, and is experimenting in aero-dynamics. removing the toy from an 18 mos. old doesn't carry any weight b/c he's in the out-of-sight, out-of-mind stage. when he throws something, try saying calmly, "we throw balls" and give him a soft, nerf-type ball to throw. and since it's developmental, he WILL outgrow it, so be patient, this will not last much longer.

Thanks for the advice. I've tried being more relaxed with DS and have gotten a positive response. At first he would just look at me when he went to throw things but since I haven't been giving him so much "attention" for it he has stopped throwing so much. He and I are both happier and the days go a lot smoother.