Try a bath and give it a day. Nathan is mostly breastfed but is supplemented with formula from time to time. He has only gone 24hrs without a BM... I panicked but he ended up pooping just before I called his pediatrician. Call doc and see if maybe the formula has too much iron in it. When I was a baby my mother had to use low iron formula and actually had to give me suppositories so I could go potty. Make sure you're drinking an adequate amount of water as well for breastfeeding.
I'm pretty sure it's constipation because he struggled like he's trying to have a bm. I pushed his legs to his chest yesterday when I noticed him struggling and it helped him have a nice sized bm, but it didn't work the same way today. He's breastfed but supplemented with formula because we've had many issues with the bfing.
Are you sure he's constipated? If breast fed babies can go days between bowel movements. If formula fed it could be constipation and you could try a warm bath. If the bath doesn't work you can try putting vaseline on a qtip and insert just the tip in his rectum and move it around a small amount, anal stimulation almost always will make an infant have a bm but be careful about how far you insert it.
Maybe try bending his knees toward his chest that can help some.