5 weeks old baby and does sick?

my son is 5 weeks and he does sick from his mouth and also from his nose.his eyes go still and he chokes realy badly.i canot leave him alone.his body stiffens upand until i dont pick him up and masage him hel be stiff.why does this happen?im awake day and night constantly looking at him incase he is not choking i get realy scared.he does not always do sick from his nose but when he does its scary