Hello, The Pickup transmission has developed a severe leak of fluid.It will leak continually, whether the truck is running or just sitting.If left parked the transmission oil will leak from above center of the unit.I was underneath the truck yesterday and noticed the pan gasket weeping some and tightened the pan bolts. No other work has been performed on the transmission.Where else would the oil leak be originating from?

yes but it depends on how long it has been this way

Could have been a stuck thermostat or a faulty water pump.
You may now have a leaking head gasket.

That would depend on which transmission you have. add transmission fluid while running in park until you reach the marks on the dip stick. You may have to check it several times as the torque convertor will suck up some of the fluid as you fill it.

Smoke may indicate a transmission leak, with fluid getting on the exhaust system. Puddling underneath when adding fluid seems to confirm that.
We can only hope that it didn't burn up any internal parts.

A few questions that might help you to get an answer:
1) Does the dipstick indicate full, or close to it?
2) Can your son be more specific as to where the fluid is leaking, and how much? Whether it is from the front or back part of the transmission, or elsewhere. Also, is it leaking out a LOT of fluid when he adds some?
3) Is the pickup parked on level ground? If not, which end is pointed up?
4) We are talking about a rear wheel drive P/U, not a 4x4, right?
5) Has he tried to put it in L1 if the pickup has it?
6) He knows to add the fluid while the pickup is running, right? Just making sure.

Sorry for all of the questions. Hopefully it will narrow down the possibilities.

You are going to need to have the transmission rebuilt.

Check your trans fluid level (car must be running to perform this properly). If it is low, there may be a leak, check the transmission oil lines on the radiator for leaks. If ok, try flushing then replacing the transmission oil and filter. Its not guaranteed to work!! Over heating does damage the transmission. Reason is, your trans oil runs through the radiator to keep it at a controlled temperature. When the engine overheats, the radiator becomes extremely hot which in turn heats up the transmission oil instead of cooling it. This may or may not cause severe damage!!

Front transmission pump.For a Chevy S10 its located behind the torque convertor,This pump supplies the torque convertor and it service as a pressure supply system, Just because the pump is in the immediate area it does wear out and develops gaps and is unable to supply transmission fluid to the system. When the transmission warms up the fluid thins out.Sitting for 5 minutes allow the fluid to cool just enough to run again
P.sthis remind me of what happen to me I stuck on the side of the road ,half mile and took 2 hours to drive back

Sometimes the injectors develop leaks as a result of minute particles of dirt sitting on the seals. Also check your fuel pressure regulator, if that is leaking it will make the engine run rich and this could look like an injector problems. Make sure the fuel filter is renewed, it could be that rust and crud from the tank is being let through. If possible also have your tank cleaned, after 20 years a certain amount of stuff is bound to be contaminating it.

It sounds like when you had the trans service don't he didn't do something right. He might have put the wrong fluid in. Does the trans make noise? Sounds like the trans is going in your truck.

either your transmission is completely out of fluid and needs more to work, you need a fluid change, or need a new transmission.