Not sure why you are assigning the ascendent to him. Did he ask the question?
As you phrased it, Should my boyfriend buy.... then the 7th house should represent him.
Mercury retrograde is your boyfriend, while the car is the 3rd house, Saturn.
Mercury approaches a trine to Saturn however it turns direct before completing the aspect.
There won't be a purchase made.
Moon from his 2nd house of money approaches a trine to Neptune on the car's 2nd house of value. He will find out the car is not worth the money asked.
Then moon opposes pluto in the 2nd, he will break it off because of the money, as mercury goes direct.