This answer comes from the angle of "it's about all players and GM having fun" and provides some "If-then" waypoints.
- He's not able to attend each session.
- He's not into the role play element as much as rest of players.
- GM Fun appears to be at risk.
Issue 1 is Presence
When he is not present, you can have one of the other players run his character for that session, rotating any time he doesn't show up among those willing.
If you are all friends who generally get along, this can remedy the "the game won't be cancelled because he didn't show up" concern.
(Bonus, another player gets to do MORE role play! :) )
Anecdote: Our current D&D 5e group with six adults, most with kids, resorts to this. It works for us.
Are the other players OK with that? Talk to all players.
If all are happy with it, good, press on.
If not, then the GM option to "play him as an NPC when not there" is a way to proceed if you all still want him in the mix when he's available.
I have seen that work but it adds to GM task load. See point 3 on GM Fun.
Issue 2 is Style
Minimal role playing desire, mostly action desire.
Ask yourself this ... "Is it me, the GM, who is bothered by this?" See below on GM fun.
Are the rest of the players getting bothered by this or not?
If not, press on and play. If he role plays less he may miss out on some XP that those who role play more/better earn.
Is there risk? Yes.
Are you incentivizing role play? Yes. (Seems to be one of your objectives).
Is he free not to? Yes.
We each find fun in different ways.
- If the other players are annoyed by this, consistently, and you get this feedback in public or private, you have to talk to all of the players.
If you initiate discussion about this in a group session, he may feel like he's being ganged up on. (In a certain sense, he'd be right as he's being called out).
Before initiating a group talk to address conflicting expectations, you address it in a one-on-one with him first. You also should speak to him honestly regarding any GM Fun issues you are facing.
Issue 3 is GM Fun
Is it your fun that's being curtailed?
GMs don't do it for the pay. GMs Need Fun Too. (I sense that your fun is being impacted by how you phrased the set up. Apologies if I read between the lines too much and got this wrong).
If the core person annoyed by his lack of role play is the GM, not the other players, the one-on-one session with him may help you both find a middle ground, or arrive at a joint conclusion that "this marriage can't be saved."
Since you profess to be a lenient GM, it seems unlikely that this will turn into a personal confrontation. You try to be accommodating. That usually works up to the point that the GM stops having fun.
As long as you are having fun, you're getting most of it right.
When you stop having fun, something has to change:
Game, game style, or players.
Best wishes. Have fun.