"No this is not possible in midi....but you can easily use an aux jack and attach it between the headphone port on your keyboard and the speaker port on your pc and record the track through the mixer. That's how I do it".
Not unless he has a microkorg or any other synth like that, as it can actually be used as a midi controller, and so can the Akai MPK49.
Do you have a multichannel hardware mixer/audio interface? If so, you cane take a jack from the synths, and put it into your interface. For example, I have a scarlet 2i2 by Focusrite. I can have two synths going into the scarlet at one time. If you have more than two inputs, then you can definitely put them into separate tracks in FL. To do so, you record the input signal being emitted by the synth (via the jack), and just assign each audio file to the mixer within FL. For example, if you have a Microkorg, then just name the recording MICROKORG-1 (microkorg being the synth and 1 being the channel).