To meet him for the first ttime, attend the Friday party, there he will ask you to meet him later, finish the party and proceed with the night in yunas room or end it (your choice) then skip the next day until night comes and click "start night" which will then give you several options: go outside, living room, yuna's room, go to sleep - pick go outside and you should see another prompt asking you to either go running or meet with Yosuke - this will get you to meet up with him
If you already did that, then it means you need to get the nanotransistor for him from the mad science student (random school event during the day) and wait for the next party to occur, afterwards do the same steps above to meet up with him for the 2nd time