Welcome to the forum.
The hair loss is, in itself, a cause for concern. The hard skin (crusty?) sounds like a fungal infection, but a parasitic infection could also be to blame. The white specks are perhaps more likely to be flakes of skin, as opposed to a visible parasite such as lice, since most parasites live in the depths of the hair shafts.
It is best to treat for both parasitic and fungal problems. I recommend you see a guinea-pig-experienced vet for treatment, and take both the female and the affected male along as they will both need to be treated.
The vet should give Ivermectin - ideally three injections, 10-14 days apart. This will deal with any mites.
For the fungal, an antifungal shampoo is advised such as Malaseb or Nizoral, although if the skin is as hard as you say you will likely need to moisturise the skin for a number of days prior to bathing in order to get the crust off.
When you say his tummy is hard, do you mean his actual abdomen, or just the skin on it? A very tight and "springy" feeling of the abdomen suggests bloat, which could effectively have been brought on by the fungal infection. If the pig ingests the fungal spores which have caused the skin condition, the infection can become systemic, which would eventually affect eating and digestion and lead to problems such as weight loss and bloat.
I advise getting in contact with an experienced vet as soon as possible. If you can give us an idea of your location we may be able to recommend a good vet for you.
Also, please separate the male and female. This forum does not support intentional breeding, we advise everyone to separate intact males and females in order to prevent pregnancies.
