Breastfeeding & Baby weight query

[flipflopmopatop]

Hey,
Baby breastfeeding at least every 2 hours in the day and has a 5.5oz bottle of expressed milk at night (my husbands treat of being able to feed our son). He is 12 weeks and hasn't gained weight since 9 weeks. Could it be the heat? He's been sick a few times this week on top of things, he usually isn't. Debating adding a formula feed as he won't wake or feed if not hungry.

If he won't wake or feed for BF, how are you going to add in FF.

What do HV say? Is this weighing at home or in clinic?

What is nappy output like?

Tongue tie? Paced bottle feeding?

More info needed please

The main question is is he having plenty of wet nappies?

Hi,
I'd swap one breast feed with formula (premade for speed) and express whilst my husband fed as to maintain supply. My husband does pace feed when he does a bottle.
HV hasn't answered her phone, but they are not doing any home visits in our area. Only have home scales but have repeated the weight several times across the hour and matched each time - we weighed him before his 9week GP appointment and it matched what they had so scales reliable enough.
My son definitely has a lip tie and slight tongue tie, but as with my first, they won't cut it due to him gaining weight (or so he was).
Nappy output is plenty wee, one poop a day.

Can you top up with formula after rather than switching feed entirely. Seems counterintuitive to switch rather than supplement