Regarding his weight- Does he follow the same centile always? If so you need not worry.
If there is dip in centiles over time- then its a matter of concern.
Regarding his frequent respiratory infections- does he attend any day care? If so kids can catch up infections often. And in such kids upto 6 to 8 episodes of upper respiratory infections can happen in 1 year.
Just give him a healthy diet with lot of fresh fruits, citrus fruits, vegetables, nuts etc.
Ask his doctor for a micronutrient
supplement- as sub clinical micronutrient deficiencies- can present with frequent infections and failure to gain weight.
You may also fortify his diet with high calorie substances like oil/ ghee/ peanut butter/ cheese etc. This will help him gain weight.
Regarding the polycystic kidney- if his mother carries the abnormal gene- there are 50% chances that the gene will be passed on to her son. And the manifestation depends on the type of gene inherited- dominant or recessive.
Its better to have him screened for the problem- Ultrasound
, urine examination will be needed.
Hope my answer helps you.
Wish your grandson good health.