It's perfectly normal for a 17 month old to not say "papa" and only say "mama".
The timeline for a toddlers language development varies.
Even him saying at least "mama" for now as well as the other sounds like the animals make is a good sign.
My 17 month old would only say "mama" even when he would be referring to his dad.
But by 18 months he started to say "papa" as we continued to tell him that it was "papa".
It can take awhile for some toddlers to learn a new word and associate that word with what it means.
Continue encouraging your 17 month old to say "papa" and speak to your 17 month old frequently and use simple language.
Point to objects and name them, repeating the words often, like in this case, point to papa and repeat the word "papa" some more until he eventually gets it.
You should only be concerned if your toddler is not responding to their name, not pointing to objects or body parts, not talking at all or not using gestures, not imitating actions or if the toddler is having difficulty in understanding of simple instructions.