It is not easy to tell whether he is in a serious condition at this point without proper investigations.
Alright, since you are curious on that aspect, let me mention some PROBABILITIES which will remain UNCERTAIN until promen otherwise.
First, to check on the liver condition, liver functions are measured as the indicators of how the liver is doing. The liver enzymes that are usually checked include SGPT,SGOT, Alkaline phosphatase and may include LDH or GGT. (I'm quite not sure which result are you referring to as "supposed to be 80 and his was 40".
Probability #1. Liver infection such as Hepatitis C can cause some blood clotting problems which will make a person high risk to easy bleeding. Bleeding can occur in any part of the body. With your son "throwing up blood", any gastrointestinal bleeding can be considered as the source of the bleeding. If the ulcers had just started and are not yet extensive, he will have minor bleeding at the start which is not enough to cause an immediate death but that will be enough to cause some anemic effects. If left untreated, the severity can increase and he can end up having severe bleeding which can led to shock and death.
Probability #2. Liver diseases such as hepatitis C can cause liver damage which can result to portal hypertension. As the consequence, it can cause ESOPHAGEAL VARICES (dilated veins in the esophagus). The dilated veins can rupture and that is one of the reasons of "throwing up" blood. Mild damage on the veins can cause mild bleeding at the start. But when the condition is uncontrolled with further increase of portal pressure and other when other precipitating factors will be involved,a large vein can rupture and cause massive bleeding. Without intensive management, the person can die in an instant or very soon. Supposing, esophageal varices is the cause of throwing up of blood of your son, it is not a good sign as a major bleeding is unpredictable and can occur any time, and it is very possible to cause death.
Possibility # XXXXX With your son having recurrent infections, we can consider immunosuppression. That can be attributed to an existing chronic liver disease but as a drug user, HIV infection needs to be ruled out. With different infections he got, there is a possibility of having chest infection such as TB. Chest infection can be suspected as the cause of "throwing up blood". It will not cause an immediate death as the disease usually progresses with some time. If left untreated, the disease can become intensive which can lead to massive bleeding and breathing problems. Yes, he can die from that situation.
Possibility #4. With immunosuppression, he can have sort of different kind of infections which when left untreated can progress into more serious condition. He can gradually deteriorate, can have sepsis or other body organs failure, which can end up to severe complications leading to death.
As a straightforward answer to your question, we cannot tell for sure. He is in a situation that maybe not so serious now, but there is no guarantee to tell he is safe based on what he is going through. As previously mentioned, depending on the extent of the disease and the cause of "throwing up",massive bleeding can occur anytime from now, and if that's the case, yes you can possibly lose him.
The only way to tell his current situation is to have investigations done - blood tests, chest X-rays, ultrasound, culture tests,endoscopic procedures, etc. Only by then, we can at least tell the outcome or prognosis of his condition.
I hope that helps. Thank you.