DS2 is now 18 weeks and has been EBF until now. All of my DC's have been dinky (all three born well over due date and all under 7lbs.)
Mixed fed DS1 after pressure from HV at 4 weeks. DD had bottles at night from 8 weeks so DP could help feed. DS1 stayed on bottom of charts and eventually 9th by 6 months, DD maintained her own line off the bottom.
DS2 has only gained 4oz in the past 4 weeks 
so gone from 0.4 ish to off the bottom. I was genuinely surprised at poor weight gain as he's developing well in every other way and feeds well too. Holding head strongly, very alert when awake, and engages well with other DCs. And sleeps well too (6/7 hour stretch at night). Also 4-5 wet nappies during day and 1-2 dirty.
HV was obviously very concerned at weight gain and pushing formula. We were going to slowly introduce anyway as I'm going back to work in 2 months so DP will have DS2 all day.. But I hate being told by HVs what to do following experience with DS1 when moving to mixed feeding did nothing except make me feel like BF failure.
Am obviously aware of poor weight gain and will pay attention and introduce bottles as planned , but am I missing something? Should I be more concerned? Just want to BF as long as possible provided it's the right thing for DS2.