Is it a bad sign that he is doing this suddenly ?

Until you two have been dating for at least 1-2 years, there is NO assurrance that love will grow once the initial infatuation phase has died out.
And the infatuation always DOES die out. It may take only a few dates, or it may take 2 years of steady dating.

So he is losing interest. This happens the majority of the time when we are dating. Emotionally-whole men take about a year of steady dating before they are ready to commit. Don't expect "2 months" to mean anything when it comes to a relationship lasting.

As for this constant contact, this is NOT psychologically healthy ... not for the individuals NOR for the relationship.
It indicates an inner poverty .. a lack of accepting and liking our OWN company .. a need for someone else to constantly distract us from our inner unhappiness. And no one EVER remains happy in love for long ... when the infatuation dies out, we go back to being NO happier than we had been when we were single.

Especially with guys, they do not put the relationship as the central focus of their life. They have buddies they hang with, they have plans for their future. YOU need to do the same for your life, and not cling to needing constant attention.

More than that, guys are into Action, not BondingCommunication. They want to DO things with you .. not talk/text. And since dyslexia and spelling problems are much higher among males than among females, there are some guys who HATE writing or texting because they have anxiety about it and they aren't good at it.

The more you go for constant texting, the more you start to annoy the guy as his infatuation starts to fade out.

BE WHOLE ... like yourself, your own company, your envisioned future as a single person. ONLY then are you ready to make love last.
Get a life. Spend time with your friends. Set goals for your career future and work towards them. Do NOT hang around hoping this guy - or ANY guy - can make you happy.

And do NOT live in fear that he will lose interest. MOST guys WILL lose interest. And your only defense from heartbreak is being whole and being happy. That way you CAN love without it hurting you.

As for his promise about "serious soon" .. don't pay attention to that.
It happens or it doesn't.
He MIGHT be just telling you that to keep you from making a scene or being a pest.
He might actually BE busy creating his future and sees you in it. But even if this IS the case, there is no guarantee he will continue to feel this way. If you two had been dating 2 YEARS, the odds are good he will stay interested. But 2 MONTHS? Most likely NOT .. just way too soon to know how it will turn out.

Btw, an emotionally-healthy female, one who IS able to make relationships work long-term .. doesn't get attached until she has been dating the guy at least 6 months. She understands that the first while is NOT the eventual reality of the relationship, and she IS whole within herself, so she can afford to wait and see how things develop over time.

I think you may be putting to much pressure on him...