I had the exact same situtation. When my husband died (2 years ago today)
I had to find a job & move into a small apartment. It was really hard and I knew it was going to be a huge adjustment.
I made sure I was on the ground floor with a back patio so I can leave the door open when I'm home, and put down 'fake' outdoor carpet out there that looks like grass. I also have a 10 x 10 tent (100 sq. ft) to shade 1/2 the patio so it's not so hot on their feet. (I'm in texas).
My girls had a hard time at first adjusting but I have a wee pad in the apt too. So they have the pad - the patio.... plus walks. It took about 2 months for my youngest to really 'get it' but she did. They've done really well.
You may want to keep pads in the apartment in several places - and also when he does use the 'fake' grass - make sure you throw a parade and give him treats.....the praise will make him want to use it again.
If you can, try taking him out to the grass the minute you all wake up and stand there till he uses it - he may get the message - once he goes - lots of praise and run for the treats
edited to add - try taking the fake grass out of the box if you can and just lay it flat - that may be intimidating him