dress code
 

Is it acceptable for men to wear short pants in the evening ?
Is there a respectable way to dress?

Where? And to do what?

You wear whatever you want in a communist country where people can hardly find decent clothing. Yes no bikini or thongs allowed except on the beaches, you probably going for dinner. There is no 5 star hotels like in Asia where they do not allow shorts or flip flops.

There are resorts in Cuba where they suggest a dress code for dinner (long pants, shirt with collar) but I haven't really seen it enforced.

Yep they don't want to upset the tourists.

I'm assuming you're from North America, so a good rule of thumb is to dress as you would at home. Things are a bit more casual in Cuba but, as you mentioned, it is nice to show respect to your hosts and other guests by not dressing as a total slob. I wouldn't suggest flip flops for an evening dinner or show (although some people will wear them), but nice shorts are probably fine. However, if you're outdoors in the evening, long pants may be a good idea to keep the mosquitoes off, so I'd throw a pair in my suitcase just in case.

We ate at paladors in Havana. I think shorts were fine.

Paladares

You won't really find mosquitoes in Havana, but maybe a couple just in some of the beach towns with mangrove swamps nearby. The government sprays pretty aggressively.
If there were mosquitoes (which there are generally not), note that many buildings don't have screens or windows that close, often just shutters.

You should wear a guayabera - untucked.