Board Thread:Manga/@comment-27821571-20160923192030/@comment-31088533-20170119051038

Here are two reasons that I thought of:

1) To hide the titan. All of the Reiss family know what he looks like and might even know who he is. They'll be looking for him but they never will because Eren ate him. And they'd never think that he gave his son the titan since it would be better to fulfill his plans on his own.

2) Grisha might not have had that much time left since he can only live for 13 years and Eren was already around 10 so Grisha's time must be up or close to it.

He already learned his lesson with Zeke and trying to force his plans and ideals on his child but Eren wants to learn about the outside world and free humanity so the Founding Titan wold be the tool that he could try to use for that. I bet he was planning on informing Eren about everything in the basement since he was eager to help humanity but Marley's soldiers attacked too soon so he couldn't explain everything to him in time since he was probably being chased at that time.