One suggestion....I know someone who did this with some of their fathers ashes.
Purchase a locket, have the funeral home put some of the ashes in the locket, and then have it sealed. I believe the funeral home, or creamatorium will do the whole thing...it was only about $100 for this person I know...but prices may be very different.
Your son would ALWAYS have a part of his father with him this way!
My condolences to you and your son.