Your best - and perhaps only - possibility for making contact with anyone who served with him would be the division's veterans association:
34th Infantry Division Association 7700 N. W. Beaver Drive Johnston, IA50131-1902
Very important! The association, being a private organization, will normally be able to provide info only on past or present members or other individuals who have been in contact with the association for one reason or another. Many of these organizations are curtailing activities (or disbanding altogether) as memberships decline (I had the phone number of a former secretary of the above but it's been disconnected - apparently another one's "bit the dust"). When you make contact, inquire as to whether there's a 135th Regimental Association (and whether their still active). Also, if they still circulate their newsletter, ask to run a locater message. Company M was the heavy weapons company of the 135th's 3d Battalion. This may be fortunate for you in that there's a good chance his squad or section was attached to another company (I, K or L), which increases the possibility of your locating someone who served with him.