Unfortunately, I grew out of my Thomas the Tank Engine love by the time I was 7 so I can't be of much help.
In this way, I'm biased when I say "no" to the party theme. My reasoning would be, as freesia stated, that it might be beneficial to him now and in the future. Keeping the theme will only alienate him from friends and peers, especially now that they're getting older. Thus, I would agree with the private party idea.
Though, if you don't want to hold him a private party, you could give him a Thomas the Tank Engine related gift afterward. Then, if possible, explain to him why he can no longer have Thomas themed parties but can still retain his love for the franchise through play (Play, being essential to a child's development). Similarly, I had a teddy bear I still kept and played with throughout my teen years. I'm nearing 22 and am still fond of it.
It doesn't have to go away. Instead, try to let it evolve into another form.