Well, you need to sit down with your boyfriend and talk!
He's made some assumptions here - he thinks that because you already do your own horse, it will be 'no trouble' to do his as well. You need to explain that you're not happy with this, and why.
There is a balance to be had. Don't be a doormat - but do try to find a compromise where you both feel that you are pulling your weight and taking a fair share of the work.
Is he paying the yard for his horse's livery or are you? If he can't scrape together £96 for the vet, then I have to wonder how he's going to pay for farrier, feed, tack, hay in the winter etc etc.
Get up, get going, seize the day. Enjoy the sunshine, the rain, cloudy days, snowstorms, and thunder. Getting on your horse is always worth the effort.