what "word" can I use to tell someone that the job is not up to his qualifications and standard. The job in fact is below his standard.
subpar means to fall short of a standard.
In your context:
This job was subpar.
This is valid for a minor underperformance all the way up to a major one, depending on tone and context of delivery.
Other words you can use instead include:
substandard, unsatisfactory, insufficient, shoddy
The first is usually relative to 'a' standard, rather than their own personal standard as you perceive it to be. The middle two are judgements made relative to, in the first case, your own judgement, in the second the necessary standard for the job to serve its purpose. The last implies that the job was completed carelessly or sloppily.
If you want to explicitly soften the statement you could also say:
This job was not up to your usual (high?) standard.