EBF - weight

I'd start with, make sure there's a problem with weight before worrying. Some babies are slender especially if you are slender. If your pediatrician doesn't see progress and is concerned then they'll tell you. It could be that the child is tired at the end of the day and a bottle is easier than the breast too. I pumped and nursed. But if you have a particularly high metabolism, a bottle here and there may be exactly what the baby needs..
That said, I had a 99 percentile long 50 percentile weight so he was a bottomless pit until the percentiles evened out. But breast milk was way easier for him to gain weight on as it's way more caloric, and it also doesn't fill them up like formula so they can eat more frequently which helps with putting on weight. My son was tired at the end of the day so it was way easier for him to drink out of the bottle so I pumped and he drank out of a bottle. To preserve antibodies, I stored pumped milk in the fridge and warmed bottles in mild warm bath.

Personally, my first question would be why do you think he should gain weight? Is he not gaining at all? Is he changing percentile a lot from birth? Is he growing in other ways? Developmental milestones, head circumference, height, etc.
weight isn’t always the whole picture. I personally would just breastfeed more or pump and give bottles of breastmilk. I would not incorporate formula unless I did not produce enough milk to satisfy baby when hungry

I had supply issues and supplementing formula did help immensely. I would breastfeed and then offer her a bottle after and her weight came on really quickly

Be prepared, if all your LO has ever had is breastmilk, the formula may give them some tummy troubles the first few times they have it