What Causes Persistent Low Grade Fever?

Hi sir my name is Shameer I’m a staff nurse as almost 5 yr experience with dialysis and MICU, now I’m working in a home care with neuro patient ( semi paralyzed) ryles tube (4 mnth left every month changing) past history strok ,diabetic (controlled) 73 years old . Now his problem is everyday fever the last 25 days it will present in 8 hrs below 101degree fever is starting at almost 2 pm to 10pm and yesterday onwards timing is changed to morning 7 o clock sometimes it will get normal temperature without help of dolo 650. Recently we checked blood RE, U RE, LFT , RFT everything perfect without albumin (2.95) and alkaline phosphate (163) so pls help me what tests wants to do more and what is the problems of this fever thank u