I mean, at 12 weeks, their sleep is all over the place. I certainly wouldn’t be wasting money on a sleep consultant, but what I would be doing is reading up on normal baby sleep.
At 12 weeks, I personally wouldn’t expect a baby to nap in a cot. I mean, some magical ones do. And I wouldn’t expect them to nap at set routine times. They nap when they’re tired, usually after a feed, on you or in the pram or on a drive in the car. It’s still pretty much, short ish awake window, feed and sleep at this stage.
What are you doing to get him to sleep at bedtime? I don’t think it’s unusual for bedtime to take 1.5 hours at this age, depending on what your set up is. Much easier if you’re cosleeping and can lie down with them and roll away. But putting them down asleep is hard. They don’t really go to sleep on their own in a cot without being held and rocked at this stage. I would expect nighttime sleep in 1.5 to 3 hour cycles at this stage.
Maybe if you can describe what the problem is, someone can give some advice. I had one without a dummy and one with. At this age, it helped with sleep. If it’s not helping, get rid of it. If it is, keep using it. It may be what you need to do most is adjust your expectations and also make sure you are trading off with your partner if you have one. Dads can get up in the night too, even if they need to work the next day. You’ll both have to do it if you return to work at the end of mat leave, so no reason it can’t be done now.