Hullo...
Here are some things you'll need to work out in order to play this character.
Age...
He needs to be at least 18 synchronize with the G1 storyline. 17 years ago, their would not have been anyone old enough to give birth to him as all the adults were wiped out 18 years ago.
Powers...
How can he throw it? How accurate is it? How many times a day can he do this? What are the limitations of this power?
History...
No.
It is very unlikely... make that so unlikely that it is inches away from impossible for him have tried any attack on Dream Castle and lived longer than say a month afterwards.
The number of Silver Dream's forces are enough to fill a town. Plus she has many Shadow Walkers (who are assassins that can be "one" with shadows), magic users, spies... you name it. Chances are he would have been blasted into space moments after his first aggressive move.
Silver Dream is also seen as the "good guy" to the normal population, he would be more than just wanted... he would be continuously hunted even by simple townsfolk.
If you want him to be a "freedom Fighter" why not have him go up against the powerful Slave Masters? We do have those. They enslave orphaned children and young mares... putting the foals into slave labor and the mares... well you figure it out.

We did say that the game isn't for children in the disclaimer.
Family? Is there any history regarding his family that you'd like to share?