A president cannot legalize marijuana or write or change any laws, that is not within the realm of his authority, that is the job of Congress so do not fool yourself into believing that President Obama can and will legalize marijuana.
He could of course ask Congress to write such legislation and then he could do what is within the realm of his authority and sign the legislation into law but his powers are limited in such cases to asking Congress to write legislation he might want and if they do and if they write a final version that he would accept then he would naturally sign it into law.
Do I expect that to happen? No.
In the 70s I felt positive that by the 90s, if not slightly sooner, marijuana would surely be legal but I was wrong.
Now I highly doubt that I will live to see the day that marijuana is legal in the U.S.