This baby needs to receive supplementation. It is best that the baby receive this via lactation aid because:
The baby is still on the breast and breastfeeding. Babies learn to breastfeed by breastfeeding. Mothers learn to breastfeed by breastfeeding. The baby is still getting milk from the breast thus helping increase the mother’s milk production. The baby is not likely to reject the breast as he would if he were supplemented by bottle or by any method not on the breast. There is more to breastfeeding than breastmilk; the baby and mother are in close physical contact.
One way to introduce the tube is to insert it while the baby is at the breast as in this video clip.
The other is to line up the tube with the nipple and latch the baby on the breast and lactation aid tube at the same time.
Note the position of the baby:
The baby’s chin touches the breast but nose does not touch. The baby covers more of the areola with his lower lip than his upper lip. The baby is slightly tilted up towards the mother. When your baby has fed from both breasts and is not getting much milk flow (mostly nibbling at the breast—see video clips of babies drinking and not drinking), and your baby is still not satisfied, it is time to supplement.