My 9 month old son has 82% efficent kedney on left & 18% on right, what may be consequences?

Depends on pbm. Your ? Does not list the origin of the problem. Loss of fcn can be attributed to structure/blockage/infection/ metabolism. Any or all may have differing outcomes. A top question in my mind is the potential for hypertension from changes in the weak kidney & or progressive loss of function.Close followup with your physician and a frank discussion of the future surveylance needs are in order.
More information. It would be very hard to analyze this problem without any medical history and the underlying cause of the poor renal function in the R kidney. I suggest you see a pediatric nephrologist to get more answers on outcomes. It may be necessary to have some specific renal function tests done to get a good answer in this case -that could involve surgery as well as special nuclear medicine studies. .