Lots of reasons and some solutions - brute force is to upgrade bandwidth from ISP... less expensive ideas are:
https://www.lifewire.com/lag-on-computer-networks-and-online-817370
http://www.pcgamer.com/how-to-tune-your-network-for-gaming/
If it is puzzling, ask your son... actually, he might be the best person to pursue many of these ideas.