"Actually, current Russian laws allow citizens who have completed their compulsory service, without any further commitment, to be mobilized up to the age of 40, 50 or 55"
So, "actually", that doesn't apply to a 56-year old.
"Russia also maintains a "mobilized reserve" of men who have signed up to receive periodic military training: those from this reserve with the highest ranks can now be called back into service up to the age of 70, other senior ranks up to 65, junior officers up to 60".
If he's signed up voluntarily (unlikely if he's been here for 5 years) it would be no issue to him, as a volunteer; if he hasn't signed up it wont apply to him.