I am really sorry to hear about your son.
It is difficult to pinpoint to any particular cause from the information available so far.
Sudden death in sleep is termed as 'Sudden Infant death Syndrome' and there are multiple reasons for the same. There are some metabolic conditions which place extra demands on the body during the time of illness and consequently brain/heart doesn't get enough glucose
leading to their failure.
Many illnesses don't manifest with all the symptoms on first presentation and deteriorate very rapidly in matter of days. Infections can get worse and affect breathing or heart pumping function. Many infections lead to enlarged spleen
Metabolic diseases like fat-disorders can present like this. Your doctor should be able to discuss with you in details.
Dr Tejas Shah