If there is only one person praying behind the Imam, should he stand next to the Imam in the same row, with social distancing, or should he stand behind him like the rest of the rows, if he is unable to stand directly beside the Imam or slightly behind him due to social distancing measures in mosques?

It is sunnah for the congregant (if he is a man or a boy) to stand to the right of the imam, abreast of him and close to him.

If the congregant fears that standing close will cause him illness, or if it is prohibited by regulations, or if he is unable to stand close, then he should stand to the right of the imam, abreast of him but distanced from him. He should not stand behind him, because even if closeness is excused due to an a valid reason, straightening the row is still possible, and praying behind the row is disputed regarding its validity and may even invalidate the prayer according to some scholars.

One must be careful not to stand in front of the imam. Equality in position is determined by the heels, not the tips of the toes.