Hello from JustAnswer.
The popping and cracking in the ear does not mean that the ears have gotten secondarily infected. It does mean that there is some inflammation and congestion, either in the ear itself or affecting the opening of the Eustachian tube that connects the middle ear with the back of the throat.
It is common for someone that has recently recovered from a cold to have some persistent inflammation and congestion that can persist for weeks. This can be due to allergies, but also can be non-allergic inflammation. This happens to me whenever I get a cold. The exact location of the inflammation will determine the symptoms that are present, including the nasal drainage that you describe as snot, but also the popping and cracking in the ears.
8 year olds are generally reliable in reporting whether there is any pain in the ear or a significant loss of hearing. As long as he does not have any pain or significant loss of hearing or any systemic signs of infection, such as a fever, then you do not need to be concerned about any secondary ear infection. Even with pain or decreased hearing, it may not be infected, but if these symptoms develop, it would be appropriate to be concerned about an ear infection. It is also true that even if infected, many ear infections can resolve spontaneously, particularly in older children and adults, so the current recommendations are to only use antibiotics for more severe infections or persistent infections.
Therefore, at this point, the symptoms are not indicative of an infection, but if he develops the above symptoms, then an infection may be present.
If I can provide any additional information, please let me know.