There is a long list of conditions that can cause low energy, but when the low energy is associated with some swelling of the feet and legs, it would be more worrisome for conditions affecting the heart, lungs, liver, kidneys, or thyroid.
If he is not having any chest pain or shortness of breath, it would be less likely that he is having heart or lung problems. Liver and kidney blood tests are almost always included in routine screening blood tests. However, thyroid tests are not as routinely included, although they may be done. If any of these labs were borderline in June, it would be appropriate to be concerned that they may have gotten worse. And if something was not checked, such as thyroid tests, it would be appropriate to check it.
It is also possible that swelling could simply be related to decreased activity levels because of the low energy level. The muscles in the legs help blood return to the body and heart, so when there is less use of the leg muscles from inactivity, there can be decreased return of blood that can then cause swelling.
It is also true that increased salt intake can contribute to swelling, although it is usually only a problem for people that have conditions that tend to cause swelling.
Would it be relatively easy to get a copy of the labs done in June? Some offices have a system in place for patients to access their lab work online.