It's not about him choosing to take it or not. It's about him deciding he doesn't have to do what you say. So what if he doesn't like the flavor. He has to take it for a medical reason. A doc would not have prescribed this med for a 5 year old if they didn't feel it was necessary.
I would poke it in his mouth and make him swallow it. I know, I'm a mean one. I also love using snot suckers....lol.
If you have to dilute it then do it. And sit there until son drinks the whole thing.
If it was me I'd use my kid burrito trick and feed it to them, truly I would do this and the kids know I would follow through because this is what I used to do if they wouldn't blow their nose and I got to use the snot sucker on them.
Since he's older and you haven't done this before you might need some help, strong help.
Take kiddo and play him down on top of a blanket on top of a sturdy dresser. Wrap him up like he's the filling of a tasty burrito. Arms and feet encased, he is still laying down. Then use your left arm to encircle his head and your right are to use for the med/drink. If you have a helper it's their job to maintain the burrito wrap. So that you can hold his head stable and make the kiddo's mouth open.
Using a dropper or syringe you fill it and squirt a few drops into his mouth in the cheek area between the teeth and cheek. He needs to swallow it so gently stroke his throat in a downward motion. Not hard, you don't want to make him hurt. Then refill and do it again.
He's not going to be happy about this but it will get more of the med in him than he's taking now.
What this accomplishes is a double meaning. Kiddo now knows there is worse things in this life besides the flavor of that medication. He now knows he is going to take it regardless.
So what is he going to do after you do this again a couple of time? He's going to suck it up and drink the med in the glass of pop. He's going to figure out that mom and/or dad has the power to actually MAKE him take it.
I only had to say Honey, can you go get the blanket that can completely wrap kiddo up? He won't take his medication this morning and I am not playing this game. Kiddo grabbed the glass and chugged it down. Once they figure out mom and/or dad is not going to let them get away with not taking it they suddenly can do it by themselves.
Talk to the pharmacist and see if there are other options. I'd say there were.
The liquid is 20 mg/mL and the lowest dose in pill form is 25mg. So once he's a bit older/weighs more he should be able to take the pill form.
A therapeutic dose can be as much as 200mg per day in an adult. So it's a small small tiny dose your child is getting. The liquid is mostly alcohol so that's probably what he's tasting. Did you try OJ? That might have a strong enough flavor to cover the med.
BUT a pharmacist is an expert in medication. I trust them to know more about a medication than any doc around.