4 month old hard to get to sleep- Help!

It has been getting harder and harder to get DS to fall asleep since he refused taking dummy anymore at 3 months old. At first all I needed to do was to nurse him to sleep when he ask for something to suck. Now it takes over 1 hour of nursing, rocking & singing, walking him in a sling and nursing again to get him to close his eyes, by then it's well past the 2 hours wake time. He would looked so tired (red eye brows, puffy eyes and jerky movements) but he would still wiggle and turn his head to look around when I rock him. When I nursed him he would likely kept pulling off then poo or play with my top and look around. Thought the rocking maybe too stimulating for him so I tried putting him into the cot after a cuddle, tell him ?sleep time? and then pat and sing to him, but soon he would whinge and want to nurse, if I ignore him and continue patting he the whinge would turn into scream. I also tired darkening the room but he protested. He has always been a gassy baby and now all this crying is making him even gassier.

I have considered he maybe overtired so I've tried to unwind him earlier but it didn't make any difference. Now he is definitely overtired... what went wrong? I'm running out of idea what else I can do apart from letting him cry :wall:... please please any tips or advice?