I hope you don't mind OP I had a search and can see that your baby is around 11/12 weeks old based on another thread (just to save people asking) :)
Is it the health visitor/weighing clinic who have told you his weight gain is too slow, or are you worried because of signs in your baby/your breasts? It is very common at about 3 months for your milk supply to regulate which sometimes feels like your milk supply is dropping, as your boobs stop filling up between feeds and many women experience a decrease in leaking. In addition many babies have a fussy or distractable period at this time and night time sleep can go haywire, plus there is a usual lull in weight gain, where weight gain begins to slow down a little, but this is normal, and should not be a concern unless your baby is dropping through the centiles on his chart in his red book. If they continued to gain weight at the rate they do shortly after birth they would be the size of an elephant by three.
My advice to you would be to put the baby to the breast at least once an hour during the day when you're at home, and every time he feeds offer both sides (wind or change in between if he is sleepy or refuses) and then offer the first side again before you top up. If you don't have older children (I think no?) at some point during the day make some time (I found the morning was good) where you strip your baby down to just a nappy, take your top off and just lie in bed together with you both covered with a blanket or sheet, baby on your chest. You can lie flat on your back or prop yourself up on some pillows. The baby may try to latch or may simply be happy just being close to you - you can breastfeed, make faces at each other or simply let baby sleep. If he's crying and won't latch, try again when he is calmer. This is called Kangaroo Care and it is excellent for getting the oxytocin flowing which helps your milk supply. You can also bath together if you have another adult to help you.
There is no problem with giving top ups as well especially as this will be an interim measure. If you've managed to breastfeed this far, I doubt you will need the top ups for long. For now, I would strongly recommend doing paced feeding, which means you hold the baby in a supported upright/sitting position and hold the bottle horizontal or close to, so that he has to actively suck to get the milk and gravity isn't doing the work for him. It doesn't matter what kind of bottle you use, but you should use the slowest flow teat you can find (size 0/newborn is good). You don't need to size up as he gets bigger. I did find that a small, narrow bottle works better than a fatter one. The kind which come with Medela breast pumps are perfect, this kind of thing. This feeding position lets baby take lots of breaks much as they do while breastfeeding and you'll find that it is clear when they have had enough. This way you can offer up to 30ml but it isn't a problem if he doesn't take all of it (it probably means he isn't hungry). If you're offering the breast every hour I wouldn't top up every feed but offer the top ups about as often as he would demand feeding, or once every 3-4 hours, or when he still seems hungry after you've offered both breasts and the first one again. You can skip topping up at night at this age unless you want to (e.g. he won't go back to sleep without it).
As for expressing, if you're putting the baby to the breast once an hour then you shouldn't need to but if your baby is only taking a "big" feed once every few hours then it may be worth expressing at the midpoint between - just do it for about 5/10 minutes each side and don't worry about how much milk you get out, it's for stimulation not production.
This article is a very good guide to common worries about supply plus has advice about the steps which can help increase supply and/or identify problems which may be impacting on supply, so may be useful:
www.emmapickettbreastfeedingsupport.com/twitter-and-blog/low-milk-supply-101