Aerial photographs of his property in California show a new fence which has been constructed around the swimming pool in the back garden, along with a trampoline which has been brought in, presumably for the little boy.
The 53-year-old actor has been dating Charlize, who adopted two-year-old Jackson in March 2012, for a few months and they spent the New Year together in Hawaii.
The loved-up couple are thought to have also spoken about marriage, although the actress previously said she's not keen on tying the knot with anyone because it has "never something that was important [to me]."
The pair walked their first red carpet together earlier this month the Fame and Philanthropy Post-Oscar Party held after the 2014 Academy Awards.
A source also told E! News they have been "spending night after night together and moving very fast."
They added: "It seems like it's getting serious and that they can't get enough of each other."
Sean divorced second wife Robin Wright in 2010 after 14 years of marriage, with the pair having a daughter Dylan, 23, and a son Hopper, 20.
He was previously married to Madonna for four years in the 1980s, while Charlize, 38, was in a long-term relationship with Irish actor Stuart Townsend, 41, until 2010.