Thanks for your question.
I'm afraid there's no point sugar-coating this for you - I wouldn't book anywhere or spend any money with a view to getting married on that date until you hear of the decision as to his visa. They aim to reach a decision on each application within 2-3 months of submission but they frequently overrun.
There is very little you can do to hurry them up, they are overstretched and not exposed to recourse in the event that they overrun their estimation of how long it will take because it is only an aim and not an actual obligation upon them. You cannot hold them to it unfortunately.
You can try calling to pester them but the practical reality is that this is unlikely to speed things up, it may make them look at the file but not much beyond that. If you make them aware of your position and keep the pressure on it can't hurt though.
If you spend money then you run then every penny you spend is at risk of being lost. A better view might be to wait until you hear of the application, not spend any money, have an improvised low-key wedding then renew your vows in a more substantial fashion at a later date, though I accept this is a poor substitute for what you had in mind.
It's a difficult situation but it's very much par for the course I'm afraid.