Even before Italy entered the Great War he was at odds with the Socialists over his interventionist stance. He was also highly influenced by Coradini's authoritarian nationalism and, conversely, radical syndicalism. He merged these schools OTL in fascism.
In order to keep him Socialist you probably have to keep Italy out of the war and then have him either a) manage to mend the bridges with the more internationalist, non-interventionist party, or b) have him more astute politically earlier and have him take over as the Socialist front man and drive the internationalist/anti-interventionist wings out.
The former leads to Wolfpaw's scenario, only I doubt we see Grandi or Balbo or anyone else as *Duce. Probably a continuation of the Liberal State with a few more crazy factions in the mix (the various pieces that OTL Mussie would unite into Fascism) and some notable instances of violence. If anyone takes over Mussie-style in this scenario it's Coradini's Blue Shirts.
The latter, maybe we see a Red March on Rome or more centralized *Biennio Rosso, but being by nature a Red Threat it's highly unlikely to receive the tacit approval of King and Army, leading to a short-lived Red Uprising that's brutally repressed by the Army. Perhaps Balbo ITTL leads one of the "loyalist" militias in clashes with Mussolini's Reds.