Happy Independence Day to all my fellow Americans and to freedom-lovers 'round the world!
I love the shield throw and the shield bash! And as a kid I loved Captain America. In 3.5, they had a feat chain where your man could learn to throw his shield just like Cap, but it was a lot of feats for very little payoff in the end.
Of course in old D&D, you can try to do anything you want to try. So I think it's perfectly OK for your man to use his shield to bash a bad guy.
If you use variable damage dice, a shield is probably a 1d4 weapon, maybe a 1d6. But I would go with 1d4. You can still use your shield for AC bonus when you shield bash. And you don't get an extra attack. If you use some rule that gives extra attacks, you can alternate between your shield and your other hand freely.
Maybe you give a -1 to hit because it's an unbalanced off-hand weapon. But in a pinch, it might just do the trick.
You can't throw a tower shield as a weapon without exceptional strength (it's just too unwieldy), but you might be able to throw it to trip or belay someone. Throwing it is probably a two-handed affair.
The range of a thrown shield is 1/2/3, and you get a -4 to hit because it's really not meant to be thrown.
In the case of hitting a lever or wedging open a closing door, it might be your best bet. And of course, there could be situations where you need something heavier than an arrow or sling stone to get the job done.
A thrown shield does not bounce back to you; you have to go pick it up.