Formula for a previously EBF baby

My baby boy is 10 months now and has transitioned to a bottle with formula. The reason we needed to switch: his pediatrician was worried that he’d dropped weight and we came to the conclusion my breastmilk is not efficient at this point to sustain proper growth.

Anyway. He’s been on the formula (similac advanced) since Tuesday afternoon last week. This afternoon he had a very soft BM that smelled horrendous and had a “stringiness” too it. He is just coming off of a cold like sickness causing phlegm and I read that there could be mucus in the stool from him swallowing down snot and mucus from his nose and throat.

Is it normal for his poops to smell so bad after switching to formula and could the BM be a result of an intoletance to the formula or would we have seen that in the first couple days he was on it?

I’m nervous about what might be happening in his little body and need some reassurance.