There's a deleted scene in which Tom enters the Corleone compound after Sollozzo releases him. He says to Sonny, "Boy, if I ever argue before the Supreme Court, it won't be as hard as I worked with the Turk today," or something to that effect--indicating that he faked his enthusiasm for the deal so he could escape Sollozzo's clutches and get back to helping Sonny. And, in the novel, he flat-out tells Sonny he lied to Sollozzo about how enthusiastic both he and Sonny were for the deal.
That said, Tom did urge Sonny to go for the deal--but only if the Don died. I don't believe Tom's loyalty was ever in question, then or later.