There are two ways your fiance can obtain his spousal visa once you're married. You can submit the application by post, which hopefully shouldn't take more than a few weeks and is cheaper, or you can take advantage of BIA's 'premium' service and take the application in person to a public inquiry office (
locations here), which will process his visa while he waits but costs more. Submitting by post is £395, and the PEO option is £595. You must make an appointment in advance if going to a PEO, but he'll get his visa on the day.
Once he has the correct visa, he can pretty much come and go as he pleases, but he'll still be subject to immigration control (filling out a landing card, using the queue for non-EU citizens if he's traveling alone,
and you can read this
bit of guidance for using a different queue if you're traveling together). There's really no hassle though if his visa is in order.