Baby constipated

My baby is 8 weeks old and hasn't had a poo in the past 24hrs. He's usually quite regular. He's straining like he needs to poo but nothing. He seems in pain and gets upset when straining so this is what makes us think he's constipated. We've massaged and given him warm bath but nothing, although that is helping with any trapped wind as he keeps trumping. Is this normal? Anyone any ideas how we can help him? I feel like he will feel so much better once he's had a poo. Thanks