Ds2 has a speech sound disorder and we have the Mr Tongue book. I can type it out for you if you think it would appeal to your son but Ds2 cannot begin to do any of it, although he has special needs so I don't think he really sees the point.
Some other suggestions - blowing and sucking - musical instruments can be fun, such as recorders and there are a whole variety of whistles etc that can be got here . Ds2 couldn't blow anything at first so it may take some perseverance.
Can he suck through a straw - those small cartons are good to start off with because you can squeeze juice up into his mouth to encourage him.
Placing dabs of honey/chocolate spread/jam around the child's mouth and encouraging them to lick it off - a mirror may help with this.
Licking a spoon coated with jam or something nice - hold it further away from the mouth and to the sides to encourage them to move their tongue.
Food to encourage the use of lip and tongue muscles includes breadsticks, apple, ice lollies, raw carrot and peanut butter on small crackers.
Alternate kissing and smiling at teddy or doll.
I have to say that I find it hard to get Ds2 who is also 4, to do a lot of these exercises but I don't know how much that is to do with him having special needs and how much is because he finds them hard and tries to avoid them.