Dress uniform of the 2/4 ChCh (Chasseurs à Cheval) regiment if I had to guess, I believe the Régiment des Guides wore more intricately braided dolmans and lancers had different collars. Since there is no real way of telling which of the regiments he was part of it is hard to say for sure where he might have fought but both of the Chasseurs à Cheval regiments saw combat in WW1 in battles such as the Siege of Antwerp, the Battle of Yser and Reigersvliet.

The 2nd was part of the 5th Division (Mons), the 4th was in the Cavalry Division (Brussels), if you're able to find more information about which specific unit he belonged to it might be good to start with the divisional history of the relevant unit.