Does the death of an infant son after observing the opening takbir for forty days signify the acceptance or rejection of one's deeds?

Congratulations on consistently catching the opening takbir with the Imam. The absolution mentioned in the noble hadith (definition) is attained by whoever consistently catches it for forty days in congregation, not just for the Fajr prayer. Allah Almighty accepts acts of obedience if the doer is a believer and their action conforms to the Sharia (definition). The tribulations one experiences are not necessarily an indication that righteous deeds have not been accepted; rather, they may be a sign of Allah's love for the servant. Patience and seeking reward from Allah in times of calamity carry immense reward. Congregational prayer in the mosque is obligatory for an able-bodied man.