Is wearing necklaces with the name of Allah on it permissible for men?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Gracious, the Merciful
It is not lawful for men to make use of any type of jewellery except for a silver ring. Wearing a ring is a sunnah only when it is worn at the time of necessity such as when a judge or author wears it for stamping. When it is worn without necessity, it is permissible though it will not be considered a Sunnah.
Also, a silver ring should not be more than one mithqāl (approx. 2.9 grams). That said, wearing a necklace/pendant is not permissible for men.
Only Allāh knows best
Written by Alimah Sabrina al-Faarsiyyah
Checked and approved by Mufti Mohammed Tosir Miah
Darul Iftā Birmingham
 Fatawa Al-Hindiyyah V.5 Pg. 388-389, Al-Maktabah Al-Faisal Allamah Ibnu Abidin, Raddul Muhtar, V.6 Pg. 357, Darul Fikr Beirut