This helps us just a little bit further. Of course problem remains in a way if you do not know what grade of NCO he had in the Imperial Navy. There is an enormous difference between the uniforms of for example a Oberbootsmann, a Obermaschinist, a Steuermann and again with the lower grades of NCO's such for example Maschinistenmaat, (Ober)bootsmannsmaat, Signalmaat, ... Of this last group the uniforms were quite alike and almost identical with the ones of the ordinary sailors (Matrose, Obermatrose, Heizer, Oberheizer,...)
It could be interesting if you had any clue such as one of the ships he was on, when he entered the Imperial Navy, what grade he had. Do you have anything left on him that could be of interest to identify his ship or unit at all ?
Secondly : are you sure about the Pickelhaube on the photo ? Because no Pickelhaubes were used in the Imperial Navy. A Schako in the worst case, most of all with units such as the Seebataillone, which were landbased.
It's rather difficult I know, but hey, don't give up !
You can also launch the question on my Forum that is specialised in the Kaiserliche Marine :
www.voy.com/65298/
You never know, as the biggest specialists on the subject to visit us.
Best from Regulus