As JVS Rao jee mentioned, decent chances of marriage open up from Moon-Mars period (9/7/2018 to 7/2/2019) during Moon-Mar-Rahu period (21/7/2018 to 22/8/2018) or especially during Moon-Mar-Sat (19/9/2018 to 23/10/2018) as Saturn has Venus (lord of 7th house of marriage) as nakshatra lord. Also around October 2018 Jupiter will transit to first house of your son thereby helping in pushing for marriage.
Two things to note
1) Your son is undergoing saadesati and Saturn will transit its natal position in second house as well. So his thinking and value system will be under review.
2) In his birth chart Saturn aspects 7th lord Venus; so its recommended for him to marry post ~32 years of age when he would achieve more clarity and maturity about what he wants in a relationship.
If waiting is an option, waiting for Moon-Saturn period (7/12/2021 to 9/7/2023) will not be a bad idea as his Saade Saati would be over or during final phase.