Contesting an estates igned beneficiaries.?

My grandmother left my 60 percent of her trust/estate and my Dad 40 percent. Also 311,00 in the form of Mutual Funds and annuities. There are 4 mutual fund accounts and 3 annuity accounts. She named me primary beneficiary of nearly 240K of the total 311K. My dad is upset and said he wants me to apply the same ration me 60 him 40 on the total igned accounts. As it stands the only thing that has to be split 60/40 is her house and cash in the bank. The stocks are all igned to me except for about 50K. I have two questions, Can he legally contest the stock beneficiaries, and if so, will this delay me getting them in liquid form? He is quilting me into signing a contract outside the will to relinquish monies I get to him to make it fair. I think she did what she did and that's what she wanted. I just don't want to cause a giant delay and court battle if he has a possible argument. He really showed his colors in this process by becoming very visceral.