Hi
Frequent upper respiratory tract infections are common in children under 6 years of age and the frequency of these episodes will decrease after 6 years as they will get resistance . To prevent frequent these infections frequent hand wash, avoiding crowds and people with respiratory tract infections, by taking immunizations like pneumococcal and flu vaccines after consulting your doctor. As polycystic kidney disease runs in family members screening of the child with ultrasound abdomen and pelvis and regular follow up for the monitoring of development of cysts in the kidneys is required. Consult your Pediatrician for investigation and advise.

Hi

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.