Hello rantonios,
It sounds like your baby has one of two things: 1.Harlequin Syndrome 2. Unilateral Horners syndrome or 3. Autonomic epilepsy.
Harlequin syndrome is common and not a worrying symptom. Parents often describe a clear demarcation down the middle of their child's body with one half being white and the other red or even purple. Is this what you see happening? This is a normal symptom that often fades as the child grows and is caused by a transient misdirection of messages from the autonomic nervous system. This is the part of the body that regulates flushing, sweating, blood flow, fight or flight reaction etc. Please do not confuse this with Harlequin's icthyiosis if searching on the internet.
In Unilateral Horners syndorme just the face is affected. Certain never pathways could have been damaged during the process of delivery. Here the affected side is the side that remains pale, the upper eye lid may droop, and an inability to sweat or dilate the vessels under his skin causes one part of his face to remain white, while the rest of his face is angry and red. I think this is unlikely if your child's whole face becomes red at the start of each episode.
I think the third possibility, though extremely rare, is the most important to rule out. Autonomic epilepsy is also called diencephalic epilepsy. During the moments when he tenses his face he could be having a short seizure. Seizures in children are very brief and easy to miss. It is worth videoing these episodes on a mobile phone and making a note of his behavior before, during and after (is he drowsy, does he wet himself, does he drool, stare etc). This information will help you paediatrcian decipher whether your baby needs to have an EEG to diagnose epilepsy.
I hope this answer is not to lengthy or technical. I am happy to write back to clarify any issues.