Does it get easier?

My son is 3 1/2 months and a very smiley baby.  However, most of the day is spent holding him in the dark while he sleeps and feeding him (bottle).  When he is awake he has to be very entertained or within 2 minutes he starts fussing.  He seems to have signs of mouth pain which is making him even more irritable.  The only peace I get is walking him in the stroller or sitting still in the dark holding him.
  I need encouragement.  When will this end?