Can my fiancé enter UK more than once on a multiple-entry Standard Visitor visa?

The UK Standard Visitor Visa is a multiple-entry entry clearance that runs for 6 months. During that time the holder can come and go much the same as a non-visa national (e.g., Americans, Canadians, and so on).

The big issue arises when someone's visit pattern implies that there has been a change of circumstances since the visa was issued. For example, if someone claims to be employed full-time (studying full-time, etc) at the application stage and then proceeds to visit for 3 or 4 months at a time or builds up 5 or 6 visits in a short period of time, it raises a flag. The flag says that the person's visit pattern is out-of-whack with their stated lifestyle.

Immigration Officers are trained to spot those flags and follow up on them. A change of circumstances will get them annoyed because the rules say that the person will declare them. So if there's been a change of circumstances your fiance should always take care to declare it up front. IO's will also get critically annoyed if they think the person misrepresented their lifestyle at the application stage. This would also be indicated by a person spending lots of time in the UK because people who are employed do not usually do that.

Can he return again for another holiday in a few months or will this be an issue?

The best rule of thumb is to look carefully at what was portrayed at the application stage. If nothing has changed and the visit sync's up with the person's lifestyle and circumstances then of course, enjoy another visit (or several visits). Your fiance would say something like...

The purpose of my visit is to maintain a relationship and there are no changes in my personal circumstances between now and when my visa was issued.

It needs to be an honest statement. As always, interaction with a British Immigration Officer is largely governed by personal impact and articulation skills. Everything (literally everything) is in the discretionary zone.

Also I like to advise people arriving on Standard Visitor Visas to have all of the original evidence they used to hand. They should also have accommodation plans to hand. Sometimes it's helpful, sometimes it doesn't matter.