I was told give my 2yr old benylin (dextromethorphan) cuz he's 48 lbs and 3.5ft tall. it says 6 and up 10ml so i only want to give 5ml cuz hes same as 6 but still only 2?

Not the weight: That matters , it's the age, for the maturity of the CNS and the unpredictable reactions to these drugs in toddlers and patients younger than age 6. You didn't specify what formulation you wanted to use. Moreover your son needs to be checked for his weight: way above, he is tall too, but still he is obese, kindly get referred to a pediatric endocrinologist, good luck