The few possibilities that this might be are all dermatitis, since they all affect the skin. Even though the bumps do not now look like insect bites, the problem may originate in one. The presence of itching may suggest this is an allergic reaction to the insect saliva; which would result in a secondary infection including inflammation and discharge. This kind of reaction could also be caused by an irritant in his environment. Has any part of this environment changed recently?

The last possibility is that this is a bacterial infection of the hair follicles (this is similar to acne). This is often related to rubbing of the skin with dirty objects such as blankets or tack, or heavy sweating during work. All of these conditions can be treated effectively with antiseptic shampoos and occasionally antibiotics. Consult with your vet for a suitable course of treatment.