For most moms, an increase in supply will be seen within 24-72 hours of starting the dose. You can increase both herbs to 4x daily to see if that makes a difference. If you decide to try and wean off to see if there's a difference, then decreasing slowly (for instance drop to 2 caps 3x a day) and waiting a few days to see if there is a change in production, is a good way to go. If you notice a decrease in supply, just go back to what you were taking.
BTW, herbs ARE medications. Over-the-counter medications and herbs can be helpful or harmful depending on the situation. There is a long history of safe use of these herbs in breastfeeding mothers, but they are being used as a medicine. They are likely to have fewer side effects than Reglan. Domperidone is considered quite safe by most breastfeeding professionals, but is tough to get in the US due to the FDA regulations. Either prescription medication is being used off-label for increasing milk supply.
You mention starting solids. Unless your DS is truly not getting enough milk to satisfy his needs, solids are going to replace milk calories in his diet and are unlikely to lead to signficant changes in weight gain. Most solids are less nutrient and calorie dense than breastmilk, so adding solids in an attempt to increase weight gain is likely to backfire. If he's showing signs of readiness for solids and is around 6 months of age, then by all means he should be introduced to them, but slowly and with a focus on maintaining breastmilk as his main source of nutrition.
Perceived slow weight gain in breastfed babies is one of my big pet peeves. Breastfed babies growth normally slows after about 3-4 months. Are you looking at his growth on the WHO growth charts, or the CDC charts? Google CDC growth charts to find their site; the CDC itself recommends against the use of their charts for children under 24 months. Most US physicians are still using the older CDC charts, however. Dropping lines on a growth chart is not uncommon. Weight at birth is more about placental placement and mom's nutrition than genetics; if he was a big baby who gained well in the first few months, but his genetics are for a smaller size, then he's going to drop.