Thank you for contacting BibleAsk.
I am very sorry for the loss of your dear grandson. Suicide is often surrounded by mystery. Only God knows the reason and motives for your grandson’s suicide, therefore people should not assume his destiny. God loves His children unto death (John 3:16) and we know that He will be the most compassionate and merciful judge even to those that committed suicide.
Now to answer your question: how can you know that your grandchild is in heaven?
According to the Bible, people receive their rewards and punishments at the second coming of Christ and not at death. Let’s look at the Bible evidence:
Peter the apostle writes, “The Lord knoweth how to deliver the godly out of temptations, and to reserve the unjust unto the day of judgment to be punished” (2 Peter 2:9). The wicked and the righteous are reserved somewhere until the day of judgment arrives.
Where are they reserved? Jesus gives the answer:
“Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice, And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation” (John 5:28, 29). Our Lord made it clear that all would be reserved in their graves until called forth in the resurrection to receive either eternal life or death.
Jesus clarifies that the judgments and rewards are given only at His coming and not at death, “For the Son of man shall come in the glory of his Father with his angels; and then he shall reward every man according to his works” (Matthew 16:27).
Again, Jesus confirms, “Behold, I come quickly; and my reward is with me, to give every man according as his work shall be” (Revelation 22:12). This means that none of the righteous dead have gone to heaven or the wicked have gone to hell. All are waiting in their graves for the judgment and the end of the world.
The old testament also teaches that people receive judgment at the resurrection: “But man dieth, and wasteth away: yea, man giveth up the ghost, and where is he? So man lieth down, and riseth not: till the heavens be no more, they shall not awake, nor be raised out of their sleep” (Job 14:10, 12). And when will the heavens be no more?
The Bible gives the answer, “The day of the Lord will come … in which the heavens will pass away” (2 Peter 3:10 NKJV). The heavens pass away on the day of the Lord, when Jesus returns to the earth to judge every one according to his works. Until then, the dead do “not rise” from the grave.
So, your grandchild is now resting in the grave awaiting to be resurrected at the Judgement Day.
For more on what the Bible says about the state of the dead, check the following link: https://bibleask.org/bible-answers/112-the-intermediate-state/
In His service,