A BP of 104/76 mm/Hg is not all that low. Here is a high to low list of what is what for adult BP:
CATEGORY - SYSTOLIC / DIASTOLIC
Hypertension - 140 mmHg or higher / 90 mmHg or higher
- 120–139 mmHg / 80–89 mmHg
Normal - 90–120 mmHg / 60–80 mmHg
- 90 mmHg or lower / 60 mmHg or lower
As you can see from the list 104/76 mmHg is very good. It's smack in the middle of normal. It's even more impressive at his age.
You were right to be concerned and seek help for him when it dropped to 88/71 mmHg. However, since the doctors only gave him an IV I suspect he had become dehydrated. That can cause a drop in BP. Be sure he drinks enough liquids (water is best)
For more information on Low Blood Pressure see the links below.