I am Dr Mbong.
Abcesses once they collect ideal should be drained, pus taken for culture and antibiotic sensitivity analyses after which appropriate antibiotics should be prescribed.
If and how the abcesses should be drained will depend on where and how deep they are located. Please do see your dad's physician or specialist for better advice. Well tailored antibiotherapy (which take into consideration also your dad's past medical history) will take care of the abcesses as well as the myositis
Hope this helps. Wish your dad good recovery.