Developmental delays in 6 month old baby

Hi We have noticed following development delays in my 6 monyh old son 1/ he has just not laughed loudly (many be once or twice in all these months) 2/ whenever we call out his name he doesn't even look (usually recongnises other sounds like clap, whistle etc and finds the source rightly) but we make so many attempts to talk to him in those baby sounds) he doesn't respond or even look. He doesn't make an eye contact while trying to engage with him. And otherwise when you walk in the room he clearly makes an eye contact but just doesn't show any when you are trying to engage 3/ doesn't utter any sound yet. Like ma, ba, da etc 4/ he doesn't crawl yet. Though walks in the walker already and rolls over Other milestones like reaching out to objects, recongnising faces, sitting without support or even walking with support and recognising faces are quite well met. Should I meet a doctor and if yes then which doctor?