It's definitely possible that he could have seen the blast, but not from the Indies. He may have been imprisoned in one of the two camps near Nagasaki, which had many Dutch POWs. /u/restricteddata might have some better sources, but I'll lay out the basics here. This website is ─ though not professionally built or maintained well ─ staggeringly complete in the amount of information it has at hand in one place. As always, be careful and vet all sources.

The two closest POW camps to Nagasaki were Fukuoka #2 and Fukuoka #14, the latter being just 1.15 miles from ground zero. About 300 Dutch POWs arrived in Fukuoka 14 in 1943, and they were joined by others in the latter two years of the war. When the bomb dropped, some were being forced to dig a tunnel and thus were spared from the blast. Others were enslaved at an iron foundry and likewise survived. Only eight POWs were immediately killed. Here's an eyewitness account of the blast from a camp survivor.

Fukuoka #2 was about five miles across Nagasaki Bay from the city itself, and everyone at the camp had a front-row seat to the blast ─ far enough to be unharmed but close enough to have an unobstructed view. The first POWs arrived at the camp in late 1942 and included British, American and Dutch soldiers captured in the East Indies.

Both camps have rosters of the men present when they were liberated in September, and if your grandfather was there, he should be listed.