Hi Nicki,
My 10 month old is the same - has days where he will not take a bottle at all. My DS is a big boy 22lbs, so if he does not have all his milk one day, it does not do him any harm.
One thing I have noticed is that when he goes off his formula, or only takes a minimal amount it is due to teething. We once had a bottle strike that lasted 5 days cause he was teething and it hurt to suck.
One thing I did do was try rubbing some teething gel on his teeth about 10 minutes before he was due for his bottle - sometimes this helped and sometimes it did not.
If he did not have much milk in a day, I would wait until he was asleep and make up a bottle and give it to him as a dreamfeed even if it was midday - at least I had my mind at rest that he had a bit of formula for him.
Like Yaya said - Has your lo always been on formula? If so what are the usual amounts that your lo would take in a day?