All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, may Allaah reward you for being keen on guiding your husband to righteousness and for encouraging him to observe the prayers at their prescribed fixed times, and we ask Allaah to reform him.
There is no doubt that it is forbidden for him to deliberately delay the prayer from its prescribed fixed time and he should undertake the means which will help him to wake up especially for the Fajr prayer. We have already clarified some of these means in Fataawa 89228 and 88163.
Hard work is not a sound reason for him to delay the Fajr prayer from its prescribed fixed time. If he is truly determined to do so, Allaah will enable him to this effect.
You should continue to advise him in a nice and soft manner and clarify to him that he may lose a great reward and may be very sinful if he is neglectful of prayer.
You should seek the help of righteous and knowledgeable people as you have already tried this and found it beneficial. If he does not respond to your advice, then you are not sinful.
Allaah Knows best.