Advice on using nipple shields

To start off, my wife is a lurker here and a regular on m.k.p.

We had our little boy on Wednesday morning 22nd August at 3:25am.

Right away, we started breastfeeding and the baby was taking in the
colostrum. We were concerned, however, because he likes to suck on his
lower lip and my wifes nipples are a little flatter than we'd like.

Our midwife suggested using a nipple sheild made by Medela. We started
using it today (day 3) and this evening her milk started to come in. The
baby has been drinking and passed a seedlike poop this evening, however
we cannot seem to get him off it and he cries non-stop until placed back
on the nipple.

I managed to get him to sleep by offering my finger to suck on but
shortly after he went asleep and I removed it, he started crying again.

While sucking using the nipple shield, he does tend to stop sucking and
either lose interest or fall asleep until the breast is pulled slightly
and he starts to sucking again.

I'm not sure what I'm asking. Maybe has anyone had similar problems?
Could it be something she ate that is making him fussy? Could it just be
gas that we've not been able to bring up? Any suggestions or advice
would be gratefully received right now.

Regards
Nigel