Answer:
The particles from the explosion will hit him, create holes in his space suit, and can also hurt him. By feeling all these things, he will know about the explosion.
Step-by-step explanation:
Explosion happened behind him so, he wouldn’t be able to see it. Moreover, Sound needs a medium to travel but, in space there is no medium. So, we would not able to listen the sound of explosion. Hence, he will feel the explosion when particles from explosion will hit him and hurt him.