Constipated 5 month old?

My son is EBF and we started with purees about 2 weeks ago. I didn't want to start food until 6 months, but his doctor told me to mix his new reflux medicine with baby food, so we went ahead and did that. He's only pooped once since then and it was very thick and he strained really hard and cried. He's also been waking up from naps and sleep in the night crying and arching his back. I've tried a warm bath, bicycle legs, gas drops and nothing has helped. Any advice for a sleep deprived mom? I miss my sleep and my happy baby! All he does is cry now. 😢