Ok, this bugged me so I ran a check and "if" this is to be believed, here is Lennon's will. From what I understand it's leaving Ono as executor and the one in charge. It mentions children from the marriage only and I think it's leaving it up to Yoko.
It also has a paragraph toward the end saying, in short, anyone contesting the will will receive nothing. No mention of Sean.
Now, in fairness, Ono could have given him "his" share, I don't know. Maybe she did.
Still, this is kind of morbid being able to look someone's will up on-line. Shame on me.
TIS
Lennon's Will