The AAP and WHO recommends no solid foods prior to 6 months. If a mother is unable to provide enough breastmilk, and cannot get donor milk, then infant formula is the recommended substitute for breastmilk.
And it is not true that many breastfed babies are hungry and cannot get enough breast milk. The vast majority of mothers are capable of making enough if not more than enough milk for their babies. Many mothers THINK they have low supply, when they do not, and some mothers do inadvertently reduce their supply due to not nursing frequently enough, unnecessary supplementation, certain medications, etc.
In this case, the baby may have a milk transfer issue as evidenced by the recent slowed weight gain and painful latch. A poor latch MAY indicate poor milk transfer which MAY lead to low supply. But especially since baby has gained great up until very recently, I wonder what may have changed to slow the gain down or if it is even an issue, weight gain is seldom entirely consistent. Certainly something to keep a close eye on however.
Doing a few before and after nursing weight checks may give you an idea if there is a milk transfer issue. But this is something a Lactation consultant or someone who knows how to do these accurately needs to help mom with. Plus you need a very sensitive scale.
Have you tried breast compressions? http://www.nbci.ca/index.php?option=...tion&Itemid=17