So what? He showed respect and that is enough.
When President Obama was first elected and was going to meet the queen of England then the question came if a US president needs to bow down to the queen. An answer was no. The media, especially the UK based, made a lot of noise but Mr. Obama didn't bow down to the queen. In fact, Mr. Trump pated the queen on her back at his first visit which raised many eyebrows.
Similarly, a PM of Pakistan need not bow down to any Kings or Queens. Yes, Pakistan needs few dollars to keep its economy running but it the Saudi Royal family that needs Pakistan so that they can cling on to their thrown.