Even at 20 some people can't wait to get out into the world and others are timid and cling to the familiar. For your son he would be trvelling away from home and doing someting unfamiliar and giving up a secure job even though the pay is dreadful.
I would ask him if he had been granted the time off how would he feel about the opportunity with his Uncle? Where does he see his life going in the next 5 years?
Get the paper and see how many jobs there are offering the wage he has now and if there are many show him that even if he does not retain his job at the petrol station then as he is a good worker there are other jobs out there paying the same low wage who would love to have him. It is also very likely that his boss would have him back in a heartbeat and he is just pressurising him because he is a good worker and good workers are hard to come by.
Point out to him that opportunities like the one offered by his uncle are rare ones and at 20 he is young enough to take risks and try new things when we get older it is the opportunites we let pass us by we regret. This is an opportunity to do something new with the comfort and support of family.
At the end of it all he is a grown up and capable of making his own decisions even the wrong ones. As a parent you can only advise.