I translated this literally as 'he is at his changing' ie, 'he is changing'. Wrong. Maybe it needs 'fein' to mean 'he is changing (himself)'?
It could be 'He/It is being changed'. 'He is changing' would be Tá sé ag athrú. If you wanted 'himself' then you'd need the féin
What does this mean? For example - a dad changing his son's nappy, he is changing him. Or is it more like influencing him, for example since he met him all they do is drink all day, he is changing him.