Hi R., congratulations on breastfeeding your baby! I have been a lactation consultant for 20 years. Before adding supplements or a pumping regimen to your busy schedule, let's first see if there really is a problem. The fact that your breasts are feeling less full does not in itself mean that you are not making enough for the baby. This may simply be the result of your production being very well synchronised to his needs. It is normal for a breastfed baby to feed frequently, even at this age, so that does not indicate a problem. We are what anthropologists call "continuous contact" species, and our babies are biologically designed to feed frequently due to the composition of our milk. And most importantly, he appears to be satisfied after feedings. If he were not, he would let you know, by nursing more frequently or longer. Weight gain is another excellent indication of adequate intake. Babies at this age are generally gaining about 6 ounces per week. From 4-6 months, breastfed babies usually gain 4-5 ounces per week. Solids are not recommended until 6 months of age.
I am sure you are very busy with three boys! You didn't mention how old your other sons are. Take a look at your schedule: is he allowed to end the feeding or do other distractions lead you (or him) to end it before he might really be done? Does he use a pacifier a lot? Ideally most of his sucking needs should be met at the breast, to stimulate supply and maximize his intake.
Don't fixate on the growth charts at the pediatrician's office, because these are not based on breastfed babies. As long as he is gaining and growing in length and head circumference as well as weight, he should be fine.
Has any other change occurred that might have caused this difference? Feeding less at night, for example? Your taking some kind of medication, like the Pill? If not, just relax and keep nursing. If he has a growth spurt he will increase frequency/length of feedings. As long as he appears happy and satisfied, just relax and enjoy!
I do encourage you to find your local La leche league group. It is a great way to find very knowledgeable support for breastfeeding and learn about normal breastfed baby behavior. You can always call a leader to address specific concerns even if you don't attend any meetings. Good luck!