60ml drinker

DS has been BF since birth and has a top up bottle of formula before bed which we've never had problems with. He's usually had around 100mls before bed.

In the last few days I've started doing a bit of mixed feeding and doing 2-3 feeds from the bottle. At these feeds, he's only been taking a maximum of 60mls, very rarely he will take more than this.
With BF'ing he will only take one side and feed for a maximum of 5-10mins.

He had good weight gains in his first few weeks, gaining 100-200grms a week. The last few weeks that's decreased. Should I be worried that he's only taking so little?

We've also had a few issues with him taking the bottle and I'll be returning to work in a few months time. It's becoming a little stressful!